feat: pull private images without a hand docker login; surface the pull error

Design merged first: monky-design-docs !225 (doc 24 §4a). Pairs with monky-tenancy!40, which
copies the estate-wide read-only Harbor robot into each environment's own prefix and marks the
manifest entry `use: registry-auth`.

- That entry is not an env var (it would otherwise land in .env and therefore in every container's
  environment). The agent parses it — JSON, or `username:password` with the new `registry_host` —
  and writes `<state_dir>/docker/config.json` 0600 in a directory it owns, with an explicit
  DOCKER_CONFIG pointing the docker CLI at it. The unit runs as monky-deployd, so a `docker login`
  by a human or by root is invisible to the agent: that is what made env-dev-01 look like it had no
  credential at all after the operator had just logged in.
- `compose pull` failures now carry the registry's own message ("no basic auth credentials",
  "manifest unknown", DNS) into the journal and the report instead of `rc=1`.
- Tests: both credential shapes, the refusals, 0600/0700 modes, idempotent rewrite, and that the
  runner never silently falls back to a human's $HOME.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KLB7jieMNRkTsJ2epr4Ds1

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "monky-deployd"
-version = "0.1.6"
+version = "0.1.7"
 description = "Monky backend pull agent: checkin -> bundle -> lease -> OpenBao -> docker compose -> report, over the ziti mesh (MONKY-ADR-0028)"
 readme = "README.md"
 requires-python = ">=3.12"