chore: quarterly gh search inventory for legacy diff-triggers format

## Context

Legacy `diff-triggers` trigger parsers stay through v1.0.0 ([#212](https://github.com/bcgov/actions/issues/212)). Runtime `::warning::` annotations are per-run only — they do not give fleet-wide visibility. **No telemetry code in the action**; use `gh search code` instead.

## Tracking playbook (manual or scheduled issue)

Run periodically (e.g. quarterly, or before any v2.0.0 parser-removal discussion):

```bash
# Parenthesized bash-style (primary legacy format)
gh search code 'triggers: (' --owner bcgov --filename '*.yml' --filename '*.yaml'

# Delimiter-separated single-line (secondary legacy; harder to grep — spot-check READMEs)
gh search code 'triggers: backend/,' --owner bcgov
gh search code 'triggers: backend/;' --owner bcgov
```

Record results in a comment on this issue (or a linked tracking issue). **Removal is safe when parenthesized search returns zero** (or only archived repos / intentional test fixtures).

## Also search suite repo before v2.0.0

```bash
gh search code 'triggers: (' --repo bcgov/actions
gh search code 'triggers: (' --repo bcgov/quickstart-openshift
```

## Tie-in

- After [#217](https://github.com/bcgov/actions/issues/217) (suite doc scrub), `bcgov/actions` search should hit only `diff-triggers/README.md` deprecated note + `self-check.js`.
- Revisit parser removal at **v2.0.0** only when org search is clean and `quickstart-openshift` template uses multiline.

No code changes required for this issue — process only. Optionally add a recurring GitHub issue / calendar reminder.

Part of [#212](https://github.com/bcgov/actions/issues/212) / v1.0.0 stabilization ([#209](https://github.com/bcgov/actions/issues/209)).