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>

.gitignore

@@ -11,6 +11,9 @@ __pycache__/
 # Rust
 /target/
 
+# Dev daemon data (isolated from the installed heph; see `mise run dev`)
+/.dev/
+
 # Linter caches
 .ruff_cache/