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/api/alembic/versions/.pi-map.index.md

@@ -2,12 +2,14 @@
 dir: apps/api/alembic/versions
 
 ## role
-Manages database schema evolution and version control for the API application using Alembic migration scripts.
+Database schema versioning and incremental migration management for the API's relational data model using Alembic.
 ## parent
 index: apps/api/alembic/.pi-map.index.md
 map: apps/api/alembic/.pi-map.md
 ## children
--
+- apps/api/alembic/versions/.ruff_cache
+  index: apps/api/alembic/versions/.ruff_cache/.pi-map.index.md
+  map: apps/api/alembic/versions/.ruff_cache/.pi-map.md
 ## files
 - 0001_initial_schema.py
 - 0002_refresh_tokens.py
@@ -51,6 +53,7 @@ map: apps/api/alembic/.pi-map.md
 - 2026_06_14_104415_add_tool_type_home_directory.py
 - 2026_06_14_182955_fix_pi_agent_home_directory_mount.py
 - 2026_06_15_090500_remove_pi_agent_explicit_repo_mount.py
+- 2026_06_19_113000_remove_pi_agent_workspace_symlink.py
 - 398082499c30_add_tool_config_fields.py
 - 6fc7bfcf199f_merge_remove_is_builtin_and_add_config_.py
 - 86cec91fdb00_merge_profile_resolver_and_workspaces_.py
@@ -69,5 +72,7 @@ map: apps/api/alembic/versions/.pi-map.md
   read: 2026_05_24_220141_add_startup_command.py, 2026_05_29_remove_lsio_command_override.py
 - change versions config
   read: 0003_user_configs.py, 0009_tool_configs.py, 0013_add_config_profiles.py
+- explore versions subdirectories
+  index: apps/api/alembic/versions/.ruff_cache/.pi-map.index.md
 ## dirty
 -