chore(z-ai): retire harness — no active subscription

Removes harnesses/contexts/z-ai/v1/. Mirrors the agent-runtimes
companion PR — the z-ai harness was never cut over to ESO and
there's no active subscription. Revival path documented in
agent-runtimes planning/future/providers F49.

Removed:
- harnesses/contexts/z-ai/v1/{harness.yaml, init.sh}

Updated:
- CLAUDE.md — drops z-ai from the 'no secrets in this repo'
  context list. Retirement note added.
- gitea-admin/v1/init.sh — comment ref to z-ai removed.
- gitea-https/v1/init.sh — comment ref to z-ai removed.

No composites layer z-ai (verified via grep across composites/);
no model registry entries reference it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

harnesses/contexts/gitea-https/v1/init.sh

@@ -12,7 +12,7 @@
 # inside an init.sh-generated helper script, which silently failed because
 # the helper runs as agent and the ESO file is root:root mode 0400. The
 # stage-and-helper-points-at-stage pattern matches gitea-ssh / minimax /
-# z-ai / anthropic-cloud-paul-oauth.
+# anthropic-cloud-paul-oauth.
 #
 # Rotation handling: per-container init. Ephemeral container agents
 # always pick up the latest mounted token. Long-running sessions need a