fix: make check optional on doctor group actions

[rubberduck203]

Summary

• check on a ScopeDoctorGroup action is now Option<DoctorCheckSpec> instead of required, so omitting it entirely gives the same "always run the fix" behavior as the existing check: {} workaround — matching what the docs already claimed.
• Both maintainers agreed on this exact direction in the issue thread; this just implements it.
• Regenerated the JSON schemas (check dropped from required); docs/static/schema is a symlink to schema/ so the docs site picks this up automatically.

Backward compatibility

check: {} continues to behave identically — it still deserializes to the same DoctorCheckSpec { paths: None, commands: None } internally, and parse_action's unwrap_or_default() produces the same value whether check was omitted or explicitly empty. Pinned by a new unit test (omitted_check_and_empty_check_parse_identically) and confirmed by the existing run-always-required fixture/integration test, which still uses check: {} and passes unmodified.

Test plan

• cargo build
• cargo fmt --check
• cargo clippy --all-targets
• cargo test (249 tests, including a new unit test pinning check: {} vs omitted-check equivalence, and an integration-test fixture now exercising the omitted-check path end-to-end)
• Manual smoke test of the exact MVCE from Correct way to create a fix that always runs? #153 (action with no check key) against the built binary
• Independent QA pass (tests/lints/coverage/mutation testing via gusto-rust:rust-review) plus a workflow-backed adversarial code review — no unresolved findings

Closes #153