fix: attach_command from one builder; no kill/start_command; stop closes a row-less session (Issue #71)

- attach_command is `herdr --session <server> agent attach <herdr name>`
  (`default` written too), built by lib_py.agents.sanitize.attach_command;
  create, resume and reconcile all use it. kill_command and start_command
  are no longer written, and status --json drops start_command.
- The two raw kill-session hints (create exit 43, id_order's orphan) print a
  stop-skill command with --herdr-session instead.
- stop_session.sh closes a session that has no registry row, herdr only and
  without --agent, when herdr runs it inside this workspace; another
  workspace's session is refused (exit 5), a missing one is exit 1, one that
  survives kill-session is exit 6. The registry is not changed.
- Docs: only the SKILL.md lines these changes made wrong; the skill block
  pins follow. Tests that asserted the removed fields keep their intent
  (issue50: create's own row; issue21: no control-root path in
  attach_command; tier2: the new form and the absent fields). S11 pins the
  shim to 4243da5, which changed it on purpose.

Refs #71

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

.agents/skills/multi-agent-mux-stop/SKILL.md

@@ -25,7 +25,6 @@ Stop an agent's herdr session gracefully, resolve and store the conversation ID,
 
 - The herdr session's recorded `pane.pid / cmd / cwd / mcp_attachments` for audit
 - The agent's on-disk conversation (claude `*.jsonl`, agy `conversations/*.db`) — so the user can `multi-agent-mux-resume` later
-- The `start_command` so a future `multi-agent-mux-create --session <name>` reproduces the same herdr spec
 
 The stop command is always **graceful by default**:
 1. Sends exit keys to the agent TUI (`/exit` for Claude, `/exit` for Agy) and waits 3 seconds.
@@ -35,7 +34,7 @@ The stop command is always **graceful by default**:
 
 ## Pre-flight
 
-`stop_session.sh` checks this itself: the YAML exists and the session is registered (otherwise exit 1), and a session already `stopped` is a no-op (exit 0).
+`stop_session.sh` checks this itself: the YAML exists (otherwise exit 1), and a session already `stopped` is a no-op (exit 0). A session with no registry row (its create or resume died before registering it) is still closed, herdr only and with no `--agent`: only when herdr runs it inside this workspace, on the `--herdr-session` server (or `HERDR_SESSION_NAME`). The registry is not changed.
 
 ## Workflow
 
@@ -59,10 +58,12 @@ bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh \
 | Exit | Meaning |
 |---|---|
 | 0 | Stopped; or already `stopped` (no-op, except with `--purge-conversation`, which still purges) |
-| 1 | From the checks before the stop, nothing was changed: YAML not found, or the session is not registered. **The registry update after the stop also exits 1** (the row disappeared from the YAML during the run) |
+| 1 | From the checks before the stop, nothing was changed: YAML not found, or the session is neither registered nor on the herdr server. **The registry update after the stop also exits 1** (the row disappeared from the YAML during the run) |
 | 2 | Bad arguments (including the retired `--mode` / `--capture-id` / `--graceful`), or `--agent` omitted and not resolvable |
 | 3 | `--purge-conversation` without `--yes` |
 | 4 | Purge aborted: the herdr session, or an agent process still holding the conversation, survived the kill chain. Records preserved: the registry row and the conversation files are untouched, though the session itself may already be down |
+| 5 | No registry row, and herdr runs the session outside this workspace: refused, nothing changed |
+| 6 | No registry row, and its herdr session survived `kill-session`; the registry is unchanged |
 | other | The registry update after the stop (YAML / DB write) failed and its code is passed through |
 
 Exits 2 and 3, and exit 1 from the pre-stop checks, come before anything is stopped, so nothing was changed. On exit 1 from the registry update, or an `other` exit, the session may already be down while its row still says `running`: check with `multi-agent-mux-status`, then run the stop again (it is idempotent).