feat: add the extra_commands registration seam for variant CLI verbs

[SunsetDrifter]

What

A wiki can register its own lint.py <verb> subcommands with the engine, dispatched by cli.main() and listed in a single unified usage string. Adds help / -h / --help.

Why

A wiki that needs its own verb (a schema dump, a custom report) had no way to register it, so it had to intercept sys.argv in its own lint.py before calling wikilint.main(). That produces two partial usage strings — the engine's, missing the wiki's verb; the wiki's, missing the engine verbs — and neither is authoritative. Any workflow or doc that treats one verb as the single source of truth is then documented by a usage string that doesn't list it.

How

• settings.BUILTIN_COMMANDS — the engine's own (verb, help) table, declared once. It lives in settings.py rather than cli.py because that module is the validation boundary and needs the reserved names to reject a shadowing registration.
• settings.DEFAULTS["extra_commands"] — {verb: (callable(root) -> exit code, help text)}, default {}, validated by _validate_commands() alongside the other extension points: dict shape, non-empty string verb, no shadowing a built-in, callable handler, non-empty help. Consistent with how extra_checks / extra_secret_patterns / skills_dir already fail fast at startup instead of mid-run.
• cli.USAGE (a fixed literal) → cli.usage() — renders BUILTIN_COMMANDS plus the registered extra_commands, column-aligned. One string, every verb, one declaration of the verb list, so usage can't drift from what main() actually dispatches.
• cli.main() dispatches a registered verb and the new help verb ahead of the CLAUDE.md wiki-root guard, so a pure-reporting verb works from any cwd. A registered command that does need the wiki root checks for it itself. Engine verbs keep the guard unchanged; an unknown verb still prints usage and exits 2.

Why it is behavior-neutral

• The shipped wiki/ template registers nothing, so it gets extra_commands: {} and its dispatch is unchanged. Pinned by a test that asserts the template does not set the key.
• usage() renders the four pre-existing verbs byte-identically to the old literal (same column width, same help text); the only difference in output is the added help row.
• The guard reordering is scoped: only registered verbs and help bypass it. check, rebuild-index, reverse-deps, coverage, and unknown verbs all still hit the wiki-root guard first and still exit 2 outside a wiki — covered by a test.

Test plan

• python -m unittest discover -s tests — 107 passed (was 91; +16)
• New tests/test_extra_commands.py covers:
  • a registered command is dispatched, receives the resolved root, and its exit code is returned
  • a registered command works with no CLAUDE.md present (the root-guard bypass)
  • help, -h, --help each exit 0 without a wiki root
  • the root guard still applies to check / rebuild-index / an unknown verb (exit 2)
  • an unknown verb inside a wiki still exits 2
  • usage() lists engine + registered verbs, and covers every entry of BUILTIN_COMMANDS (verb and help text), so the table can't drift
  • registration validation rejects: a verb shadowing any built-in, a non-callable handler, a malformed (non-pair) entry, empty/blank/None help, an empty/blank verb, a non-dict table
  • a bad registration fails at configure() (exit 2) before any dispatch happens
  • the shipped template registers no command

Note on the sibling PR

Opened alongside #11. Both are cut from main and are independently mergeable, but they touch adjacent regions of settings.py and the same README bullet, so whichever merges second needs a trivial rebase (no logical conflict).

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