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/data/skill_code_blocks.json

@@ -463,6 +463,20 @@
    "evidence": [],
    "mismatch": null,
    "note": "post-stop checks; no script offers them as a command - stage 3"
+  },
+  {
+   "skill": "multi-agent-mux-update",
+   "start": 17,
+   "end": 19,
+   "lang": "bash",
+   "sha": "63f73ade58d3",
+   "class": "c",
+   "kind": "invocation",
+   "evidence": [
+    "multi-agent-mux-update/scripts/update.sh:3"
+   ],
+   "mismatch": null,
+   "note": "entry point call (Issue #72)"
   }
  ],
  "removed": [