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

config.example.yaml

@@ -44,3 +44,7 @@ laptop_mode: false                   # true: offline exits 0 quietly; run withou
 # compose_project: monky-env-qa-02   # docker compose project name
 # docker_bin: docker
 # log_level: INFO
+
+# The registry the bundle pulls from. Used only when the seeded pull credential is a bare
+# `username:password` (a JSON credential names its own registry). doc 24 §4a.
+registry_host: harbor.tikali.net