feat: multi-agent-mux-update skill and in-place install (Issue #72)

The update is a skill (decision 4); deploy/update.sh and the installed
.mam_deploy/update.sh are gone.

- Phase 1 (installed code, update.sh): publish the update marker under the
  exclusive admission lock; refuse before any change while a running row, a
  live herdr agent in the workspace on any running server, an open job
  (unreadable JSON included, dry_run not), a live loop marker or a
  monitor.lock held by a monitor the skills did not start (U3) is there - the
  calling agent (server + pane, its row and one running job) excepted
  (lib_py/update.py precheck, decisions 1 and 7). Stop the monitor (TERM, then
  KILL and restart it if it does not end; decision 2), handle a leftover
  backup by its state (U4), back up .agents, .mam_deploy, root files, .venv,
  the installer's .mam files and the registry (SQLite backup API under the
  registry lock) into .mam/update-backup/, then exec a copy of
  install_phase.sh outside the tree (decision 5).
- The copy (no lib.sh/lib_py, bash 3.2) runs the installer in place; on
  failure it restores the backup and restarts the monitor from the restored
  tree; on success it runs phase 2, post_update.sh of the new tree, which
  converts the registry with lib_py.registry_format and restarts the monitor
  if it ran. The backup and the marker are removed at the end.
- install.sh updates in place (decisions 3, 4, 9): install_state keys are set,
  one line each; no updater is installed and an old one (.mam_deploy/update.sh,
  or a root update.sh the manifest lists) is removed; framework files the new
  version no longer ships are deleted when unmodified, kept and listed when
  modified, and dropped from base/, the manifest and asset_hashes.txt; a local
  source's .mam is not staged. remove.sh knows the new skill; CI shellchecks
  its scripts; docs point at the skill.
- test_issue8_update_state and test_deploy_layout td6/td27 run the skill.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_deploy_layout.py

@@ -48,7 +48,7 @@ class TestDeployLayout(unittest.TestCase):
         self.assertTrue(os.path.exists(os.path.join(self.test_dir, ".agents", "skills", "lib.sh")))
 
     def test_td6_td7_td8_mam_deploy_layout_and_removal(self):
-        """T-D6~D8: Verify remove.sh/update.sh installed in .mam_deploy and work properly."""
+        """T-D6~D8: Verify remove.sh installed in .mam_deploy (update.sh not, Issue #72) and works properly."""
         subprocess.run(
             ["bash", os.path.join(self.repo_root, "deploy/install.sh"), self.test_dir],
             env=self.env,
@@ -59,7 +59,7 @@ class TestDeployLayout(unittest.TestCase):
         remover = os.path.join(self.test_dir, ".mam_deploy", "remove.sh")
         updater = os.path.join(self.test_dir, ".mam_deploy", "update.sh")
         self.assertTrue(os.path.exists(remover))
-        self.assertTrue(os.path.exists(updater))
+        self.assertFalse(os.path.exists(updater))
         self.assertTrue(os.access(remover, os.X_OK))
 
         # T-D8: Execution from inside .mam_deploy directory targets parent workspace
@@ -129,17 +129,29 @@ class TestDeployLayout(unittest.TestCase):
         self.assertTrue(os.path.exists(backups_dir))
 
     def test_td27_td28_update_preserves_mam_state(self):
-        """T-D27~D28: Verify update.sh preserves .mam state files across cycles (B-5 gate)."""
+        """T-D27~D28: Verify the update skill preserves .mam state files across cycles (B-5 gate)."""
         subprocess.run(
             ["bash", os.path.join(self.repo_root, "deploy/install.sh"), self.test_dir],
             env=self.env,
             check=True
         )
 
-        # Run update.sh
-        updater = os.path.join(self.test_dir, ".mam_deploy", "update.sh")
-        res = subprocess.run(["bash", updater, "--force"], cwd=self.test_dir, env=self.env, capture_output=True, text=True)
-        self.assertEqual(res.returncode, 0, f"update.sh failed: {res.stderr}")
+        # Run the update skill (Issue #72: .mam_deploy/update.sh is gone)
+        updater = os.path.join(self.test_dir, ".agents", "skills", "multi-agent-mux-update", "scripts", "update.sh")
+        env = {k: v for k, v in self.env.items() if not k.startswith("HERDR_")
+               and k not in ("WORKSPACE_ROOT", "AGENT_SESSIONS_YAML", "MAM_JOBS_DIR")}
+        env["MAM_NO_SUBSCRIBER_DAEMON"] = "1"
+        # The update lists the live herdr agents; never ask a real server here.
+        fake = os.path.join(self.temp_dir, "fake-herdr-bin")
+        os.makedirs(fake)
+        with open(os.path.join(fake, "herdr"), "w") as f:
+            f.write('#!/bin/sh\necho \'{"sessions": []}\'\n')
+        os.chmod(os.path.join(fake, "herdr"), 0o755)
+        env["PATH"] = fake + ":" + env["PATH"]
+        res = subprocess.run(["bash", updater, "--yes"], cwd=self.test_dir, env=env, capture_output=True, text=True,
+                             stdin=subprocess.DEVNULL)
+        self.assertEqual(res.returncode, 0, f"the update skill failed: {res.stdout}{res.stderr}")
+        self.assertFalse(os.path.exists(os.path.join(self.test_dir, ".mam_deploy", "update.sh")))
 
         # Verify asset_hashes.txt and version.txt still exist
         self.assertTrue(os.path.exists(os.path.join(self.test_dir, ".mam", "asset_hashes.txt")))