[HansRobo]

Summary

Hardens the base workspace commands (R6 of the review roadmap, docs/review-and-proposals-ja.md §3.3 / §5.2). Previously cwm base offered only update (git pull + non-sourced colcon build), which left several real-world frictions unaddressed: no pull-less rebuild, manual cleanup of stale CMakeCache dirs, no colcon arg passthrough, and a build path asymmetric with the worktree side.

New commands

Command	Description
cwm base build [-- <colcon args>]	Rebuild the base workspace without pulling
cwm base clean [--yes]	Remove base build artifacts with a confirmation prompt
cwm base status [--json]	Show whether the base is built and dirty
cwm base doctor [--fix] [--json]	Detect (and with --fix delete) stale build dirs pointing at missing sources

Changes to existing behaviour

• base update / base build now source the ROS 2 underlay (underlay in .cwm/config.yaml) via run_colcon_sourced, symmetric with the worktree build path. <underlay>/setup.bash is validated up front with an explicit error instead of relying on the caller's environment.
• Colcon arg passthrough: extra args after -- are forwarded to colcon build (e.g. cwm base build -- --continue-on-error).
• cwm ws clean --base is deprecated in favour of cwm base clean; it still works and prints a notice to stderr (removal deferred to a future major).

Implementation notes

• base doctor parses build/<pkg>/CMakeCache.txt (CMAKE_HOME_DIRECTORY) and flags build dirs whose source no longer exists — the moved/deleted-package case that otherwise fails the next build with a CMake error. --fix deletes only those dirs; dirs without a cache (e.g. pure-Python packages) are ignored.
• Reuses existing pieces: run_colcon_sourced (generalised in refactor: generalize sourced colcon runner and extract changeset pipeline #16), _artifact_dirs (clean_cmd), _collect_base (status_cmd).
• Shared helpers extracted: suppress_completion in completion.py (replaces per-command lambdas in build/test/base) and _print_base_status in status_cmd.py (shared by ws status and base status).

Additive only — no breaking CLI/config changes ⇒ minor, v6.3.0 after merge.

Test plan

• 15 new unit tests in tests/unit/test_base_cmd.py (sourced build, underlay validation, arg passthrough, clean confirmation, deprecation notice, doctor detection/--fix/--json)
• Full suite: 209 passed (uv run python -m pytest tests/ -q)
• Manual E2E on a real ROS workspace: cwm base build -- --continue-on-error, cwm base doctor --fix against a moved package