fix: run tool terminal sessions as container user instead of root

- Remove compose-level user: 0:0 override from manifest_compiler.py so the
  entrypoint can start as root, fix mount ownership, and drop privileges to
  the container user internally.
- Add get_manifest_container_user() helper to resolve the manifest-declared
  container user (with uid:gid fallback).
- Pass container user through TerminalSession, TerminalManager, and the
  terminal WebSocket handler so docker exec is invoked with --user <user>.
- Update and add unit tests for the manifest compiler and terminal session.
- Record the additional root-user fix in the fix-pi-container-mount-permissions
  OpenSpec change/tasks.

Quality gates: pytest tests/unit/ (226 passed), pytest tests/services/test_terminal_manager_multi.py (7 passed), ruff check on changed files (clean), mypy on changed files (clean)

openspec/changes/fix-pi-container-mount-permissions/tasks.md

@@ -7,6 +7,10 @@
 - [x] Remove explicit repo mount from pi-agent manifest; synthesize mount in compile_compose
 - [x] Add _get_repository_mount_name() helper to derive workspace name from remote URL
 - [x] Switch workspace storage layout to /data/working-copies/{workspace_id}/{repo_name}/
-- [x] Update unit tests
-- [x] Run quality gates (pytest unit, ruff, mypy)
+- [x] Update unit tests for workspace/home-directory migration
+- [x] Run quality gates for workspace/home-directory migration
+- [x] Remove compose-level `user: 0:0` override so entrypoint can drop privileges
+- [x] Pass manifest-declared container user to terminal sessions via `docker exec --user`
+- [x] Update unit tests for container user/terminal changes
+- [ ] Run quality gates for container user/terminal changes
 - [ ] Commit and push