fix: remove stale {{WORKSPACE_NAME}} directory from container home

Older cached images still contain a literal /home/user/{{WORKSPACE_NAME}}
directory baked in by the previous Dockerfile generation. Even though new
images no longer create it, existing images leave the placeholder folder
alongside the real repo-named mount.

- Add entrypoint cleanup that removes /{{WORKSPACE_NAME}} if it
  exists before creating the real workspace target and /workspace symlink
- Update unit tests to assert the stale placeholder removal

Quality gates:
- pytest tests/unit: 212 passed
- ruff: clean on changed files
- mypy: clean on changed files

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

@@ -11,6 +11,7 @@ After implementing configurable tool container home directories, new `pi-agent`
 3. The generated entrypoint hardcodes the literal string `{{WORKSPACE_NAME}}` as the symlink target.
 4. `npm_global` packages are installed with `RUN npm install -g ...` as root into the system npm prefix, so the non-root container user cannot update them.
 5. Once the repo mount moves out of `/workspace`, the generated `/workspace` compatibility symlink is created in the image as root. The non-root entrypoint cannot replace it (write permission is required on `/`), so container startup fails.
+6. Older images baked a literal `{{WORKSPACE_NAME}}` directory into `/home/user`, which survives alongside the real repo-named mount directory.
 
 ## Fix
 
@@ -24,6 +25,9 @@ After implementing configurable tool container home directories, new `pi-agent`
    - Generate the entrypoint symlink from the runtime `WORKSPACE_NAME` environment variable.
    - Install `npm_global` packages into a user-writable prefix (`{home_dir}/.npm-global`) and add it to `PATH`.
    - Use `sudo` or root to create the `/workspace` compatibility symlink, because `/` is owned by root and the non-root entrypoint cannot replace a root-owned symlink.
+   - Start the container as root and drop privileges to the container user inside the entrypoint via `su`.
+   - Do not create mount target directories or the `/workspace` symlink in the image when they depend on the runtime `{{WORKSPACE_NAME}}` placeholder.
+   - Remove any stale literal `{{WORKSPACE_NAME}}` directory left over from older images at container startup.
 3. Update `instance_service.py` to pass `REPO_NAME` and `WORKSPACE_NAME` into manifest compilation.
 4. Update unit tests for the new behavior.