hub v0.16.0: seed artifact manifest from env; manifests image -> 0.16.0

- cmd/hub/main.go: seed the Day-0 artifact manifest from ARTIFACT_AGENT_VERSION/
  ARTIFACT_AGENT_SHA256/ARTIFACT_GOLDEN_VERSION/ARTIFACT_GOLDEN_SHA256 on startup
  (only empty fields, so UI edits stick) — same escape hatch the floor uses.
- manifests/hub.yaml: image 0.15.0 -> 0.16.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hub/CHANGELOG.md

@@ -21,6 +21,10 @@ BUNDLE slice that lets a fresh PVE box self-install the agent (no more manual bi
   managed-update floor controls, with version + sha256 fields for each artifact. `POST /configs/artifacts`
   (CSRF-protected); versions validated as bare semver (reusing the floor validator), sha256s as 64-hex,
   blank-to-clear.
+- **`main.go` (env seed):** seeds the manifest from `ARTIFACT_AGENT_VERSION` / `ARTIFACT_AGENT_SHA256` /
+  `ARTIFACT_GOLDEN_VERSION` / `ARTIFACT_GOLDEN_SHA256` on startup — but only fields the DB doesn't already
+  have, so a UI edit sticks across restarts. Same escape hatch the Phase-2 floor uses (the operator UI is
+  password-gated).
 - **Tests (`artifact_test.go`):** recorded set returned verbatim; unset → 200 empty; wrong/missing
   passphrase → 401; unknown customer → 404; store partial-set round-trip. Plus the floor-render smoke
   test updated for the new list-page data field.