fix(docs): validate runner arguments before dispatch

[injaneity]

Summary

• Problem this solves: unrecognized or empty arguments fell through to “run all,” so even --help could trigger builds and documentation writes.
• What changed: replace ad hoc argv scanning with Node's built-in parser, return early for help, and distinguish empty selectors from absent ones. Add command-level regressions to docs CI.

Related work

Fixes #3749.

RFC: not required; internal docs tooling, not a published driver API change. #3723's workflow-allocation work is separate.

Compatibility and risk

• User-visible, API/CLI/MCP, migration, permission, or platform impact: help does not dispatch; unknown options and invalid selectors fail instead of running generators. Existing documented modes remain available. No runtime driver or permission changes.
• Risk and rollback: scripts relying on ignored arguments now fail intentionally. Revert this commit to restore the old runner behavior. No new dependency or custom parser.

Validation

• Focused tests and checks: four observed red → green cycles—help, unknown option, empty library, empty changed-files path. 17 tests, routing checks, Prettier, and diff checks pass at bf48d680066c309c461072e100250dbf11207a36.
• Manual or platform evidence: real runner subprocesses with a harmless configured generator on macOS. A normal invocation proves the fixture can dispatch; invalid/help invocations must not. Compatibility coverage includes both check spellings, listing, and changed-file selection.
• Known gaps or CI still required: hosted CI pending; Windows/Linux not yet verified. No real documentation builds or historical Windows/Lume failure replay claimed.

Contributor and release checks

• The PR is focused and the description matches the final diff.
• This change does not require an RFC, or the accepted RFC is linked above.
• Tests, documentation, and platform evidence are included or the gap is explained.
• The PR title is a Conventional Commit describing the production change.
• External contributor authorship is preserved, or no external contribution is included.
• If release-tracked files changed but this is intentionally non-releasing, the no-release label is applied.

No external contribution or component release is involved; the conditional release-label requirement does not apply.