heph: dev/installed isolation — mise run dev task + install how-to

- mise-tasks/dev runs the working-tree hephd on isolated .dev/ paths (gitignored)
  so in-repo development never touches the installed daemon's data; point a dev
  nvim at it via HEPH_SOCKET/HEPH_DB.
- docs/how-to/install-heph.md: install heph/hephd from the forge (build from
  source), the lazy.nvim `dir` setup for the subdir plugin, and dev isolation.
- gitignore .dev/ and the transient .claude scheduled-task state.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mise-tasks/dev

@@ -0,0 +1,26 @@
+#!/usr/bin/env bash
+#MISE description="Run a development hephd on isolated .dev/ paths (never touches the installed daemon's data)"
+
+# Dev/installed isolation: the installed heph owns the default XDG socket+DB
+# (your real data); this runs the dev working-tree daemon on a separate socket
+# and DB under .dev/ (gitignored). Point a dev Neovim at it with:
+#
+#   HEPH_SOCKET="$PWD/.dev/hephd.sock" HEPH_DB="$PWD/.dev/heph.db" nvim
+#
+# (the plugin reads those envs), so dev edits never reach the installed store.
+
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+cd "$ROOT"
+mkdir -p .dev
+
+SOCK="$ROOT/.dev/hephd.sock"
+DB="$ROOT/.dev/heph.db"
+
+echo "dev hephd → socket $SOCK"
+echo "           db     $DB"
+echo "point a dev nvim at it:  HEPH_SOCKET=$SOCK HEPH_DB=$DB nvim"
+echo
+
+exec cargo run -p hephd -- --mode local --socket "$SOCK" --db "$DB"