releng: refactor audit workflow routing and decouple state management

releng: refactor audit workflow routing and decouple state management

Previously, workflow routing and PR state management were split between
the GitHub Actions workflow and the underlying PTL script, leading to
stale branch protection states, API pagination bugs, and blocked merges
during manual overrides.

This refactors the workflow into a centralized state machine and decouples
status reporting from the audit tool:

* Decouple State Management: Remove `--audit-label` from `ptl-tool.py` and
  move all commit status and label transitions (`releng-audit-pass`,
  `releng-audit-fail`) into a dedicated workflow reporter step.
* Non-Blocking Review Comments: Update `ptl-tool.py` in CI mode to post
  audit findings as `COMMENT` events rather than `REQUEST_CHANGES`,
  preventing stale review states from blocking PRs after an override.
* Add Step Summaries: Implement `write_ci_summary()` in `ptl-tool.py` to
  output rich Markdown diagnostic reports directly to `GITHUB_STEP_SUMMARY`.
* Atomic Override Handling: Add `setOverrideStatus()` and `executeOverride()`
  to push an immediate `success` commit status when `/audit override` or the
  override label is applied, unblocking required branch protection checks.
* Safe SHA & Label Lookup: Replace the unpaginated Bash `curl` request with
  a reusable `getPrSha()` helper and evaluate overrides directly within the
  JavaScript router.
* Handle Cancellations Safely: Map `cancelled` workflow outcomes to an
  `error` commit status rather than defaulting to `failure`.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>