test: decouple dirty semantics from statusline deadline

[SUaDtL]

Summary

Make the real-repository dirty-state test independent of the production statusline latency budget. The semantic check now uses a test-local 5-second deadline, while the dedicated timeout test continues to enforce the production 100 ms fail-soft contract.

This removes runner-contention sensitivity without changing shipped behavior.

Closes #352

Verification

• GitDirtyTests: 3 tests passed on Linux.
• Formerly flaky real-repository test: 200 of 200 repeated runs passed.
• Full hook suite: 967 tests passed with 1 intentional skip on Linux.
• Repository checks: all 25 .github/scripts/test_*.py suites passed.
• Python compile, whitespace validation, and staged secrets scan passed.
• Independent coverage review: PASS with no findings.

Tradeoff

The integration test receives a deliberately larger deadline than production. This follows the project conflict hierarchy at the test boundary: semantic coverage remains deterministic, while the separate budget test retains the actual runtime constraint.

Follow-up

A separate pre-existing clock-boundary failure discovered during verification is tracked in #355 and is not included in this patch.

[SUaDtL]

Superseded by #313, the single hackathon review surface. This PR's recorded head and changes are represented in #313's exact 16-PR / 29-commit / 126-path source manifest. PR #313 passed final exact-head CI at commit 6173b1d and remains open and unmerged. Closing this source PR to avoid parallel review; please continue review and discussion on #313.