docs(agents): clean up after a merged PR

agents/AGENTS.md

@@ -103,6 +103,9 @@ For an impactful change, follow these steps in order, scaled to its risk:
 - Documentation should be self-contained for its reader and omit irrelevant history.
 - Verify in proportion to risk. Agents MUST NOT claim a check passed without running it, or make it
   pass by weakening what it verifies (e.g. mocking the code under test).
+- When a PR the agent opened is merged, finish up as part of that task: delete the local branch and
+  worktrees it created, and fast-forward the default branch if the checkout is free. Touch nothing
+  it did not create; skip and report instead of forcing.
 
 ### Git commits