fix(ORCH-058): parametrize staging_check in --build-staging + explicit staging target

Round-3 review follow-up on c53d625 (P1/P2):

- P1: --build-staging now runs staging_check via parametrized
  STAGING_CONTAINER / STAGING_CHECK_PATH / STAGING_CHECK_MODE (default
  orchestrator-staging / bind-mount path / stub) instead of hardcoding
  $TARGET_SERVICE + the script path. docker exec runs INSIDE the staging
  container (ORCH-048 canonical: B6 registry isolation), after health,
  before exit 0. Fail-closed: any non-zero -> exit 1. STAGING only (8501).
- P2a: rebuild_staging_image now passes the STAGING target EXPLICITLY
  (TARGET_SERVICE/TARGET_PORT/COMPOSE_PROFILE/STAGING_CONTAINER) so the
  self-rebuild can never drift onto prod 8500 if hook defaults change (AC-9).
- P2b: TC-09 caller<->hook contract tests assert the ssh command carries
  GIT_SHA + BUILD_CONTEXT + the staging target and never the prod 8500 one;
  no-ssh-host fails closed.
- P3: consolidated the three duplicate README footers into one.
- Docs (golden source): DEPLOY_HOOK.md step 4 + env rows, README footer,
  CHANGELOG, Dockerfile ARG GIT_SHA="" comment, .env.example freshness block.

Validates exactly the artefact later BUILD-ONCE retagged to prod (AC-4,
ADR-001 step 3). 632 tests pass, ruff clean, bash -n OK.

Refs: ORCH-058

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile

@@ -1,9 +1,13 @@
 FROM python:3.12-slim
-WORKDIR /app
-# ORCH-58: stamp the validated git commit into the OCI revision label so the
-# deploy hook provenance guard can fail-closed on it before the prod retag.
-ARG GIT_SHA
+# ORCH-058 (Strategy B): stamp the image with the git commit it was built from so
+# the deploy hook can fail-close if a stale staging image would be promoted to prod
+# (INV-FRESH). Passed at build time via `--build-arg GIT_SHA=<sha>` (the staging
+# rebuild in check_staging_image_fresh / the --build-staging hook mode supplies it).
+# Without the build-arg the label is empty -> the hook treats it as a mismatch
+# (fail-closed). The OCI-standard key is read by `docker image inspect`.
+ARG GIT_SHA=""
 LABEL org.opencontainers.image.revision=$GIT_SHA
+WORKDIR /app
 RUN apt-get update -qq && apt-get install -y -qq openssh-client git && rm -rf /var/lib/apt/lists/*
 # git operations run as root over bind-mounted /repos (may be owned by host uid) -> trust it.
 RUN git config --system --add safe.directory '*'