fix(docker): address Ark0N's PR review on Update-Codeman.sh — fix the handoff, the build/down ordering, and default-clear the build volumes

Three blockers, all fixed and verified by actually running the script (not
just string-matching it):

1. `exec "$script_dir/Start-Codeman.sh"` failed EACCES/exit 126 on every
   checkout, since Start-Codeman.sh is committed non-executable (100644) —
   the same fact my own second commit on this branch established. Fixed to
   `exec bash "$script_dir/Start-Codeman.sh"`.

2. `down` ran before `build --no-cache`, so Codeman and every session it was
   running were offline for the entire rebuild, and a build failure left the
   stack down with nothing to bring it back — the exact ordering mistake
   Start-Codeman.sh's own "Build BEFORE taking the stack down" comment exists
   to prevent. Reordered to build, then down, then hand off.

3. The default path could throw the rebuild away: codeman-node-modules/
   codeman-dist only re-seed from the image while EMPTY, Start-Codeman.sh
   only clears them when it detects the checkout's HEAD or package-lock.json
   moved, and neither condition is true for the Dockerfile-only change this
   script exists for — so a plain `bash docker/Update-Codeman.sh` rebuilt an
   image whose fresh node_modules/dist then sat unused behind the old
   volumes. Made clearing them the default; `--keep-volumes` opts out
   (replaces the old `--volumes`/`-v` flag, which is no longer needed since
   clearing is now the default).

Smaller items from the same review, also fixed:

- The --no-cache build now derives PUID/PGID from CODEMAN_APPDATA_PATH's
  owner first, via the identical owner_of() helper Start-Codeman.sh uses
  (parity-tested) — without it, the build used Compose's default 1000:1000
  regardless of the real appdata owner (99:100 on the Unraid layout
  docker/README.md documents), and Start-Codeman.sh's own correctly-PUID'd
  build during the handoff would then rebuild those layers anyway, so the
  --no-cache image never actually shipped.
- docker/README.md's "rebuilds ... only when it detects ... moved" wrongly
  described BOTH the rebuild and the volume-clearing as conditional;
  Start-Codeman.sh rebuilds on every start, only the volume-clearing is
  conditional. Corrected, and reworded around the new default.
- --help/-h now prints usage and exits 0 instead of falling into the
  unrecognised-argument branch.
- "the ONLY named volumes this stack declares" now says docker-compose.yaml
  specifically, since a docker-compose.override.yml could add more.

New tests: PUID/PGID derivation parity with Start-Codeman.sh's owner_of(),
--help handling, and — the one that actually catches blocker #1, which five
source-string-matching tests did not — a real end-to-end smoke test: a
synthetic deployment, a stub `docker` on PATH logging every invocation, the
real script executed via a real subprocess. Confirms the real command
sequence (build --no-cache, then down --volumes or plain down, then evidence
the handoff genuinely ran Start-Codeman.sh) and that a working handoff fails
honestly at Start-Codeman.sh's own later check rather than with EACCES.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N6eadpRyqpA9PD3i139cSD

docs/docker-self-update.md

@@ -224,7 +224,7 @@ the host and the in-app path works from then on.
 
 **Resetting the build artefacts** — `docker compose down -v`, then
 `Start-Codeman.sh`. This discards the named volumes and re-seeds them from a fresh
-image. `docker/Update-Codeman.sh --volumes` scripts exactly this (plus an
+image. `docker/Update-Codeman.sh` scripts exactly this by default (plus an
 unconditional `--no-cache` rebuild, which a plain `Start-Codeman.sh` run does not
 force on its own) — see "Major updates" in `docker/README.md`.