Eliminate double towncrier run in release workflow (#199)

## Summary

- Added a new `build_quartz` Dagger function that builds the Quartz site from a pre-processed source tree (no towncrier)
- Reordered the release workflow so towncrier runs **once** on the runner, then passes the updated working tree to `build-quartz`
- `build_docs` and `build_changelog` are preserved for standalone use — `build_docs` now delegates to `build_quartz` internally

## Motivation

Previously towncrier ran twice per release: once inside a Dagger container (via `build_docs` → `build_changelog`) and once on the runner to capture CHANGELOG.md changes for the git commit. This was wasteful and fragile — if towncrier behavior changed, the two runs could produce different results.

## Test plan

- [ ] Review diff to confirm workflow step ordering is correct
- [ ] Trigger a release and confirm towncrier runs only once
- [ ] Verify the docs tarball contains the updated CHANGELOG.md
- [ ] `dagger call build-quartz --src=. --version=vX.Y.Z` should work standalone

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/199

.dagger/src/blumeops_ci/main.py

@@ -23,36 +23,15 @@ class BlumeopsCi:
         ref = f"{registry}/blumeops/{container_name}:{version}"
         return await ctr.publish(ref)
 
-    @function
-    async def build_changelog(
-        self, src: dagger.Directory, version: str
-    ) -> dagger.Directory:
-        """Run towncrier to build changelog, return modified source tree."""
-        return await (
-            dag.container()
-            .from_("python:3.12-slim")
-            .with_env_variable("TZ", "America/Los_Angeles")
-            # git is required because towncrier stages CHANGELOG.md via git add
-            .with_exec(["apt-get", "update", "-qq"])
-            .with_exec(["apt-get", "install", "-y", "-qq", "git"])
-            .with_exec(["pip", "install", "towncrier"])
-            .with_directory("/workspace", src)
-            .with_workdir("/workspace")
-            .with_exec(["git", "init"])
-            .with_exec(["towncrier", "build", "--version", version, "--yes"])
-            .directory("/workspace")
-        )
-
     @function
     async def build_docs(self, src: dagger.Directory, version: str) -> dagger.File:
-        """Build changelog then Quartz site. Returns docs tarball."""
-        updated_src = await self.build_changelog(src, version)
+        """Build Quartz docs site. Returns docs tarball."""
         return await (
             dag.container()
             .from_("node:22-slim")
             .with_exec(["apt-get", "update", "-qq"])
             .with_exec(["apt-get", "install", "-y", "-qq", "git"])
-            .with_directory("/workspace", updated_src)
+            .with_directory("/workspace", src)
             .with_workdir("/workspace")
             .with_exec(
                 [