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-create/SKILL.md

@@ -146,12 +146,10 @@ herdr_sessions:
     cmd: claude
     cmd_full: claude --dangerously-skip-permissions
     cwd: /path/to/project
-  start_command: "HERDR_SESSION_NAME=<herdr_session> herdr new-session -d -s <SESSION_NAME> -x 140 -y 40 -c <WORKSPACE> <CMD_FULL>"
-  attach_command: "HERDR_SESSION_NAME=<herdr_session> herdr agent attach <SESSION_NAME>"
-  kill_command: "HERDR_SESSION_NAME=<herdr_session> herdr kill-session -t <SESSION_NAME>"
+  attach_command: "herdr --session <herdr_session> agent attach <herdr name of SESSION_NAME>"
 ```
 
-The three commands require `source .agents/skills/lib.sh` first — `new-session`/`kill-session` are tmux-compat pseudo-commands the shim translates, and `HERDR_SESSION_NAME` is what the shim reads to route to the right isolated herdr *session* (real `herdr` has no env-var-based scoping of its own; `herdr_session: default` needs no prefix at all).
+`attach_command` is a native herdr command and runs from any shell: it always names the server (`default` too) and names the agent as herdr registered it (lower case, at most 32 characters: `lib_py/agents/sanitize.py`). Start and stop sessions only through the create, resume and stop skills.
 
 ## Workflow
 
@@ -258,5 +256,5 @@ herdr agent read "$SESSION_NAME" --source visible --lines 20   # TUI ready = age
 
 - **Resuming an old conversation** → `multi-agent-mux-resume`
 - **Killing an existing session** → `multi-agent-mux-stop`
-- **Just attaching to an existing session** → `herdr agent attach <name>` (no skill needed)
+- **Just attaching to an existing session** → the row's `attach_command` (no skill needed)
 - **One-shot print mode (claude -p "...")** → no herdr needed; use `claude-code` skill's print mode