diff --git a/docs/changelog.d/feature-branch-cleanup-task.feature.md b/docs/changelog.d/feature-branch-cleanup-task.feature.md
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+Add `branch-cleanup` mise task to delete merged branches locally and on the Forgejo remote, with a configurable age cutoff (default 30 days).
diff --git a/mise-tasks/branch-cleanup b/mise-tasks/branch-cleanup
new file mode 100755
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+++ b/mise-tasks/branch-cleanup
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+#!/usr/bin/env -S uv run --script
+# /// script
+# requires-python = ">=3.12"
+# dependencies = ["rich>=13.0.0", "typer>=0.15.0"]
+# ///
+#MISE description="Delete branches that have been merged into main (local and remote)"
+#USAGE flag "--dry-run" help="Show what would be deleted without deleting"
+#USAGE flag "--local-only" help="Only clean up local branches"
+#USAGE flag "--remote-only" help="Only clean up remote branches"
+#USAGE flag "--cutoff <cutoff>" default="30" help="Only delete branches whose HEAD commit is older than N days (default 30)"
+"""Clean up merged branches locally and on the Forgejo remote.
+
+Finds branches that have been fully merged into main whose HEAD commit
+is older than a cutoff (default 30 days), then offers to delete them.
+Uses git operations directly (SSH for remote), no API token needed.
+
+Protected branches (main) are never deleted.
+
+Usage:
+    mise run branch-cleanup                    # interactive cleanup (30-day cutoff)
+    mise run branch-cleanup -- --cutoff 7      # only branches older than 7 days
+    mise run branch-cleanup -- --cutoff 0      # all merged branches regardless of age
+    mise run branch-cleanup -- --dry-run       # preview only
+"""
+
+import subprocess
+from datetime import datetime, timezone
+from typing import Annotated
+
+import typer
+from rich.console import Console
+from rich.table import Table
+
+PROTECTED_BRANCHES = {"main", "master"}
+
+
+def run_git(*args: str) -> str:
+    """Run a git command and return stdout."""
+    result = subprocess.run(
+        ["git", *args],
+        capture_output=True,
+        text=True,
+        check=True,
+    )
+    return result.stdout.strip()
+
+
+def branch_head_age_days(ref: str) -> int | None:
+    """Get the age in days of the HEAD commit on a branch ref."""
+    try:
+        date_str = run_git("log", "-1", "--format=%aI", ref)
+        if not date_str:
+            return None
+        commit_date = datetime.fromisoformat(date_str)
+        return (datetime.now(timezone.utc) - commit_date).days
+    except subprocess.CalledProcessError:
+        return None
+
+
+def get_merged_local_branches() -> list[str]:
+    """Get local branches that are fully merged into main."""
+    output = run_git("branch", "--merged", "main")
+    branches = []
+    for line in output.splitlines():
+        name = line.strip().lstrip("* ")
+        if name and name not in PROTECTED_BRANCHES:
+            branches.append(name)
+    return sorted(branches)
+
+
+def get_merged_remote_branches() -> list[str]:
+    """Get remote branches that are fully merged into origin/main."""
+    output = run_git("branch", "-r", "--merged", "origin/main")
+    branches = []
+    for line in output.splitlines():
+        name = line.strip()
+        if " -> " in name:
+            continue  # skip HEAD pointer
+        if not name.startswith("origin/"):
+            continue
+        short = name.removeprefix("origin/")
+        if short not in PROTECTED_BRANCHES:
+            branches.append(short)
+    return sorted(branches)
+
+
+def delete_local_branch(branch: str) -> tuple[bool, str]:
+    """Delete a local branch. Returns (success, message)."""
+    try:
+        run_git("branch", "-d", branch)
+        return True, "deleted"
+    except subprocess.CalledProcessError as e:
+        return False, e.stderr.strip()
+
+
+def delete_remote_branch(branch: str) -> tuple[bool, str]:
+    """Delete a remote branch via git push. Returns (success, message)."""
+    try:
+        run_git("push", "origin", "--delete", branch)
+        return True, "deleted"
+    except subprocess.CalledProcessError as e:
+        return False, e.stderr.strip()
+
+
+app = typer.Typer(add_completion=False)
+
+
+@app.command()
+def main(
+    cutoff: Annotated[
+        int,
+        typer.Option(help="Only delete branches whose HEAD commit is older than N days"),
+    ] = 30,
+    dry_run: Annotated[
+        bool,
+        typer.Option("--dry-run", help="Show what would be deleted without deleting"),
+    ] = False,
+    local_only: Annotated[
+        bool,
+        typer.Option("--local-only", help="Only clean up local branches"),
+    ] = False,
+    remote_only: Annotated[
+        bool,
+        typer.Option("--remote-only", help="Only clean up remote branches"),
+    ] = False,
+) -> None:
+    """Delete branches that have been merged into main."""
+    console = Console()
+
+    # Fetch latest remote state
+    console.print("[dim]Fetching remote branches...[/dim]")
+    try:
+        run_git("fetch", "--prune", "origin")
+    except subprocess.CalledProcessError as e:
+        console.print(f"[red]Failed to fetch:[/red] {e.stderr}")
+        raise typer.Exit(1)
+
+    do_local = not remote_only
+    do_remote = not local_only
+
+    local_merged = get_merged_local_branches() if do_local else []
+    remote_merged = get_merged_remote_branches() if do_remote else []
+
+    # Compute ages and filter by cutoff
+    all_names = sorted(set(local_merged) | set(remote_merged))
+    branch_ages: dict[str, int | None] = {}
+
+    for name in all_names:
+        # Prefer remote ref for age (more authoritative), fall back to local
+        age = None
+        if name in remote_merged:
+            age = branch_head_age_days(f"origin/{name}")
+        if age is None and name in local_merged:
+            age = branch_head_age_days(name)
+        branch_ages[name] = age
+
+    # Apply cutoff filter
+    local_branches = [
+        b for b in local_merged
+        if (branch_ages.get(b) is not None and branch_ages[b] >= cutoff)  # type: ignore[operator]
+    ]
+    remote_branches = [
+        b for b in remote_merged
+        if (branch_ages.get(b) is not None and branch_ages[b] >= cutoff)  # type: ignore[operator]
+    ]
+
+    skipped = [
+        b for b in all_names
+        if b not in local_branches and b not in remote_branches
+    ]
+
+    if not local_branches and not remote_branches:
+        console.print("[green]No merged branches older than "
+                      f"{cutoff} days to clean up.[/green]")
+        if skipped:
+            console.print(f"[dim]({len(skipped)} merged branches skipped — "
+                          f"newer than {cutoff} days)[/dim]")
+        raise typer.Exit(0)
+
+    # Build summary table
+    table = Table(title=f"Merged branches to delete (older than {cutoff} days)")
+    table.add_column("Branch")
+    table.add_column("Age (days)", justify="right")
+    table.add_column("Local")
+    table.add_column("Remote")
+
+    candidates = sorted(set(local_branches) | set(remote_branches))
+    for branch in candidates:
+        has_local = branch in local_branches
+        has_remote = branch in remote_branches
+        age = branch_ages.get(branch)
+        age_str = str(age) if age is not None else "?"
+        table.add_row(
+            branch,
+            age_str,
+            "[yellow]delete[/yellow]" if has_local else "[dim]-[/dim]",
+            "[yellow]delete[/yellow]" if has_remote else "[dim]-[/dim]",
+        )
+
+    console.print(table)
+    console.print(
+        f"\n[bold]{len(local_branches)}[/bold] local, "
+        f"[bold]{len(remote_branches)}[/bold] remote branches to delete"
+    )
+    if skipped:
+        console.print(f"[dim]({len(skipped)} merged branches skipped — "
+                      f"newer than {cutoff} days)[/dim]")
+
+    if dry_run:
+        console.print("\n[dim]Dry run — no branches were deleted.[/dim]")
+        raise typer.Exit(0)
+
+    # Confirm
+    if not typer.confirm("\nProceed with deletion?"):
+        console.print("[dim]Aborted.[/dim]")
+        raise typer.Exit(0)
+
+    # Delete local branches
+    if local_branches:
+        console.print("\n[bold]Deleting local branches...[/bold]")
+        for branch in local_branches:
+            ok, msg = delete_local_branch(branch)
+            if ok:
+                console.print(f"  [green]✓[/green] {branch}")
+            else:
+                console.print(f"  [red]✗[/red] {branch}: {msg}")
+
+    # Delete remote branches
+    if remote_branches:
+        console.print("\n[bold]Deleting remote branches...[/bold]")
+        for branch in remote_branches:
+            ok, msg = delete_remote_branch(branch)
+            if ok:
+                console.print(f"  [green]✓[/green] origin/{branch}")
+            else:
+                console.print(f"  [red]✗[/red] origin/{branch}: {msg}")
+
+    console.print("\n[green]Done.[/green]")
+
+
+if __name__ == "__main__":
+    app()