Both from Copilot's review of #1962.
The candidate-set enumeration matched only `.ts`/`.tsx`, while
`verify:format-coverage` and `verify:typecheck-coverage` both gate all four
TypeScript extensions. No `.mts`/`.cts` exists under `core/` or
`test-servers/src` today, which is exactly why the omission would have gone
unnoticed until a shared dependency arrived through one and skewed — the
failure this guard exists to prevent. Extract the rule as the exported
`isSharedSourceFile` predicate, cover all four extensions, and pin it with
regression tests (including path-boundary anchoring, so a `core-internal/`
sibling isn't swept in).
The README's `validate` row also omitted `test:scripts` and claimed typecheck
was cli/tui only; launcher runs one too, and web typechecks via `tsc -b`
inside its `build`. Restate the row to match the actual script chain.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HDdo1rNRVQnVRRdsqSrvG3
e480fdc49d