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/features/.pi-map.md

@@ -4,10 +4,10 @@ dir: apps/web/src/components/features
 index: apps/web/src/components/features/.pi-map.index.md
 
 ## role
-Reusable UI feature components that compose domain-specific functionality for the web application.
+Reusable UI components organized by domain-specific feature areas for the web application frontend.
 ## files
 ## arch
-Feature-based component organization with domain-driven design patterns, likely combining presentational components with localized state and business logic.
+Feature-based component colocation pattern with domain-driven folder structure, separating shared/common components from page-specific feature implementations.
 ## tags
 -
 ## symbols