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)

apps/.pi-map.md

@@ -7,7 +7,7 @@ index: apps/.pi-map.index.md
 Contains the main application entry points and executable modules for the project.
 ## files
 ## arch
-Modular application structure with separate deployable units, likely following microservices or multi-app monorepo pattern with shared infrastructure.
+Top-level directory for deployable application boundaries, typically organized by target platform or deployment variant (e.g., web, mobile, desktop, CLI).
 ## tags
 -
 ## symbols