fix(tool): stop defaulting manifest working_dir to /workspace

The /workspace compatibility symlink was removed from the manifest
compiler/entrypoint in 3e59a25. Tool definitions that still set
runtime.working_dir to /workspace therefore start in an empty directory
instead of /home/user/{repo_name}.

- manifest-editor.tsx: default working_dir to empty instead of /workspace;
  update startup-script placeholder to reference /home/alex/.
- Add Alembic migration 2026_06_19_113000 that clears the stale
  runtime.working_dir = /workspace from the built-in pi-agent manifest.
- Add migration import test.

Quality gates: pytest tests/api tests/services/test_terminal_manager_multi.py tests/unit (248 passed), ruff check (clean), npx tsc --noEmit (clean), eslint (clean).

apps/web/src/components/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: apps/web/src/components
 
 ## role
-Reusable React UI components providing the application's shared visual elements, layout shell, authentication guards, and notification infrastructure.
+Provides reusable, type-safe React UI components for the web application including layout shells, data display states, iconography, code editing, syntax highlighting, authentication guards, and toast notification rules.
 ## parent
 index: apps/web/src/.pi-map.index.md
 map: apps/web/src/.pi-map.md