test(issue-45): the herdr-config tests collect on Python < 3.11; the docs say what that Python loses

tests/test_issue45_herdr_config.py imported tomllib at module level. On
Python 3.9 - macOS's /usr/bin/python3, which the venvs and the bootstrap's
python3 use here - the import failed during collection and pytest ran no
test of the whole suite.

The import is optional now, and the 29 tests that parse a result with
tomllib or expect the user's settings kept are marked needs_tomllib: on
Python < 3.11 herdr_config deliberately writes the key-only config (restore
still off, the other settings dropped, a warning on stderr), which
test_without_tomllib_restore_is_still_off covers on every Python.
Python 3.9: 22 passed / 29 skipped; Python 3.11: 51 passed.

create SKILL.md (same line, so the #42 inventory is unchanged) and the
4.8.5 entry state that building the server config needs tomllib (Python
3.11+) in the python3 on PATH, and what an older python3 loses. Checked
with Python 3.9 in a throwaway HOME: the warning reaches stderr (the
bootstrap captures only stdout) and the user's file is untouched.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

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

@@ -117,7 +117,7 @@ Why use `--herdr-session`?
 - By default, all skills target `default` herdr session socket — fine for single-workspace use.
 - By using an isolated session via `HERDR_SESSION_NAME` (or `--herdr-session`), your agent sessions are completely separated from your default user workspace, ensuring 0% interference — this is backed by a genuinely separate `herdr` session/socket, not merely a workspace label.
 - To deliberately tear down an *entire* isolated group at once (all its workspaces and agents), use `herdr session stop <HERDR_SESSION_NAME>` followed by `herdr session delete <HERDR_SESSION_NAME>` — this only affects that named session, never the default one.
-- An isolated server MAM starts does not relaunch saved agents when it restarts (#45): it runs with `HERDR_CONFIG_PATH=<your herdr config dir>/mam-server-config.toml`, your herdr config with `[session] resume_agents_on_restore = false`. Its saved panes come back as empty shells; resume closes the one of the session it resumes.
+- An isolated server MAM starts does not relaunch saved agents when it restarts (#45): it runs with `HERDR_CONFIG_PATH=<your herdr config dir>/mam-server-config.toml`, your herdr config with `[session] resume_agents_on_restore = false`. Its saved panes come back as empty shells; resume closes the one of the session it resumes. Building that file needs `tomllib` (Python 3.11+) in the `python3` on PATH; with an older `python3` (macOS ships 3.9) restore is still off, but the file holds only that key, so the MAM server runs without your other herdr settings and the bootstrap prints a warning saying so.
 - That file is rebuilt each time MAM starts the server, so a change to your herdr config reaches a MAM server only when it is next started; `herdr server reload-config` rereads the generated file. A server you start yourself (`herdr session attach`) does not get it.
 
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