v0.8.22 — cache race, pool leak, CLI exit codes/help, percent rounding, --dir isolation

A batch of empirically confirmed bugs found by adversarial auditing of the CLI's
own contracts — each reproduced before it was fixed, each pinned by a test.

Fixed

• The schema cache no longer loses writes when two processes update it at
  once. cache.py used to read the whole JSON, change one server's entry, and
  write the whole file back, guarded only by a threading.Lock — which is
  process-local. A long-lived serve gateway and a short-lived CLI command (or
  two CLIs) writing concurrently clobbered each other: reproduced with 4
  processes × 400 writes, 1,600 entries collapsed to 14. The cache is now one
  file per server (schema_cache/<sha1(server)>.json) written with
  tempfile.mkstemp + os.replace, so concurrent writers touch different files
  and neither can overwrite the other; the same 1,600-entry test now keeps all
  1,600. The old single-file schema_cache.json is still read for compatibility
  but never written. load_all_tools() is the one reader for "every cached
  schema", so bench and bench_tokens cannot disagree.
• Pooled child processes are always closed. manifest.get_manifest /
  get_server_tools built an MCPClientPool and called pool.close() after
  list_tools; a server that died mid-fetch raised before the close, leaking an
  npx/node child on every failed server. Both now use with MCPClientPool(...).
• mcptoon select "<task>" no longer crashes on a zero-match query. It
  indexed ranked[0], so any query with no hit (e.g. a Chinese query against an
  all-English catalog) raised IndexError and exited 1 with a traceback. It now
  prints "No tools found" plus tips and exits 0, exactly as search already did.
• An unknown command exits non-zero, on stderr. mcptoon notacommand used to
  print "Unknown command" to stdout and exit 0 — a typo'd step in CI or a &&
  chain looked green. It now writes to stderr and exits 1.
• An unknown option is refused, not silently ignored. mcptoon serve --nonsense printed "ignored" and ran on; a typo like --jsn for --json
  produced default output with no warning. Unknown options now exit 2 with the
  flag named (MCPTOON_STRICT_FLAGS=0 restores the old warn-and-continue).
• mcptoon <command> --help shows that command's own usage. skills --help,
  docs --help and quickstart --help printed the general help page
  byte-for-byte, so a subcommand's flags were undiscoverable. It now prints the
  command's own usage block first, then the full reference.
• Savings percentages are floored everywhere, never a fake 100%. status
  printed 640,834 → 2,499 tokens as "省 100%" (the true figure is 99.61%) because
  {gw_pct:.0f} rounded up — and it disagreed with the skills line on the same
  screen, which floors to 99.99%. The footer tools line, status, welcome and
  report now share one floored whole-number rule (footer.display_pct), and
  bench floors to two decimals instead of rounding (its skills pointer printed
  100.0% while footer-facts said 99.99%).
• status flags a skills index that lags the live catalog. It could show 464
  skills while the footer showed 469, with no hint which was current. It now
  prints "(live catalog has N — index may be stale, run mcptoon skills index)"
  when the cached live count is higher.
• mcptoon --dir X keeps every write inside X. Config and the skills index
  moved into X, but the schema cache still landed in the real ~/.cache/mcptoon,
  because cache._cache_dir() bound config.CACHE_DIR at import and apply_home
  only rebound that constant. _cache_dir() now reads config.CACHE_DIR live and
  apply_home also pins MCPTOON_CACHE_DIR.

Docs

• README's 30-second step 4 now works on a clean machine. It taught
  mcptoon call everything echo ..., but everything is only added by
  mcptoon demo --keep; a fresh install got Server 'everything' not configured.
  It now adds and calls the built-in zero-dependency demo-server, which needs no
  Node and no network.
• quickstart no longer recommends a server that can list zero tools. Its
  empty-machine guidance now leads with demo-server; the reference everything
  server (which lists 0 tools in a minimal Python image, per DEVELOPERS.md) is
  mentioned only as an aside.
• New docs/faq.md — troubleshooting for the errors people
  actually hit, including the mcptoon skills sync mis-delete gate and its
  --allow-tracked-deletes override (previously undocumented anywhere).