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-admin/v1/init.sh

@@ -13,7 +13,7 @@
 # This was previously broken: the wrapper `cat`d the ESO mount path which
 # the agent could not read. Discovered during the M22 Phase 9 audit; the
 # wrapper-script approach was never re-validated post-cutover. Stage-and-
-# wrapper-points-at-stage matches gitea-ssh / gitea-https / minimax / z-ai.
+# wrapper-points-at-stage matches gitea-ssh / gitea-https / minimax.
 #
 # Rotation handling: per-container init. Long-running sessions need a
 # future scripts.control_loop hook to re-stage between operations.