chore: standardize the ai-agent caller stub (identical in every repo)

The caller workflow had drifted between repos (run-name, stale SHA comment, trigger ordering,
different comment text) though all were functionally the same `uses: agent.yml@main`. Define one
canonical stub: make this repo's own ai-agent.yml the reference and update the README "Use it in a
repo" block to match it verbatim, noting that the on: triggers and the jobs.agent wrapper are the
irreducible minimum for a reusable (workflow_call) workflow.

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

.gitea/workflows/ai-agent.yml

@@ -1,6 +1,8 @@
 name: ai-agent
-# Thin caller so the agents work on THIS repo too (their own workflow). Same shared logic.
-# New issues opened by ffaerber auto-start @pm; @mention an agent in a comment to route on replies.
+# Standard caller for the shared AI-agent workflow (ffaerber/agents). Copy this file VERBATIM into
+# any repo that should get the agents — it is identical in every repo. All logic + scripts live in
+# agents/.gitea/workflows/; scripts are fetched from @main at run time. The `jobs.agent` wrapper is
+# required: a reusable (workflow_call) workflow can only be invoked from a caller job, not top-level.
 on:
   issue_comment:
     types: [created]