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>

README.md

@@ -38,7 +38,7 @@ bash deploy/install.sh
 - writes a starter `.mam.env` with `MQTT_BROKER=localhost`. An existing `.mam.env` is kept. If only a legacy `.env` exists, it is copied to `.mam.env` and the original stays;
 - adds `.venv`, `.mam/` and `.mam.env` to `.gitignore`.
 
-Point `MQTT_BROKER` in `.mam.env` at a broker you run (see [MESSAGING.md](MESSAGING.md) for a private broker). If `MQTT_BROKER` is not set at all, the code falls back to the public `broker.hivemq.com`. That is fine for a trial, but not for real work. `deploy/generate-env.sh` creates `.mam.env` from `.mam.env.example` when it is missing (`--force` overwrites it, keeping a `.mam.env.bak`). `deploy/update.sh` and `deploy/remove.sh` update and uninstall. `deploy/install_mam.sh` is a separate installer that copies from a local clone into another directory.
+Point `MQTT_BROKER` in `.mam.env` at a broker you run (see [MESSAGING.md](MESSAGING.md) for a private broker). If `MQTT_BROKER` is not set at all, the code falls back to the public `broker.hivemq.com`. That is fine for a trial, but not for real work. `deploy/generate-env.sh` creates `.mam.env` from `.mam.env.example` when it is missing (`--force` overwrites it, keeping a `.mam.env.bak`). The `multi-agent-mux-update` skill updates an installed workspace, and `deploy/remove.sh` uninstalls. `deploy/install_mam.sh` is a separate installer that copies from a local clone into another directory.
 
 ---
 
@@ -56,6 +56,7 @@ Every skill lives under `.agents/skills/` with a `SKILL.md` that documents its o
 | **`multi-agent-mux-delegate-job`** | Delegates a job to a session (`herdr:<name>`) or to a runner on another host (`remote:<name>`) and waits for its result. See [Delegation contract](#-delegation-contract). |
 | **`multi-agent-mux-loop`** | Plan, implement, review: an optional planner and a creator, then the selected reviewers (`--reviewer A,B`, or `--all-reviewer` for every running reviewer except the creator) until all of them `PASS` or `--max-loop` is reached. With neither option the creator reviews its own work. `--review-only` reviews work finished outside the loop. Passing reports are promoted to `.agents/reports/<session>/`. |
 | **`multi-agent-mux-orc-onboard`** | Registers the orchestrator's own session id, so session discovery never mistakes it for a sub-agent. |
+| **`multi-agent-mux-update`** | Updates an installed workspace in place: refuses while sessions, open jobs or a loop are live; stops and restores the monitor; rolls back on a failed install; converts the registry with the new code. |
 
 ---
 
@@ -195,7 +196,7 @@ sequenceDiagram
 │       │   └── *.py                   # pane_lock, stall_watch, worktree, herdr_config, ...
 │       └── multi-agent-mux-*/         # One directory per skill (SKILL.md + scripts/)
 ├── .mam/                              # Runtime state (git-ignored): sessions, jobs, audit logs, shim
-├── deploy/                            # install.sh, install_mam.sh, update.sh, remove.sh, generate-env.sh
+├── deploy/                            # install.sh, install_mam.sh, remove.sh, generate-env.sh
 ├── docs/                              # contracts/, design/, agent integration and bridge guides
 ├── nats-docker/                       # Private NATS broker deployment (submodule)
 ├── tests/                             # pytest suite