fix(ci): bind review-fix scheduler runtime to immutable workflow SHA

[seonghobae]

What

Bind pr-review-fix-scheduler.yml runtime code checkout to the immutable commit SHA of the called reusable workflow instead of mutable .github@main.

canonical_ref remains backward-compatible as an optional equality assertion: when supplied by a source-pinned caller, it must match the resolved called-workflow SHA and cannot select another ref.

Why

A caller may pin the reusable workflow entrypoint to a reviewed commit while the current implementation still checks out scheduler Python from mutable main. That creates review-to-execution drift and makes downstream claims of immutable central logic incorrect.

Security

• Resolve the trusted source from workflow job/context metadata.
• Require a full 40-character commit SHA.
• Fail closed on unexpected workflow repository, malformed context, unresolved SHA, or caller mismatch.
• Keep checkout credentials disabled.
• Add focused contract tests preventing regression to CANONICAL_REF: main.

Verification

• New source-selection contract tests.
• Existing central workflow and scheduler test suites through required CI.

Unblocks the source-pinned hourly BandScope maintenance caller.

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between 5983b41 and 0a9c0a4.

📒 Files selected for processing (2)

• .github/workflows/pr-review-fix-scheduler.yml
• tests/test_pr_review_fix_workflow_source_pin.py

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[seonghobae]

Closing as superseded by #704. GitHub's current Actions context reference explicitly supports checking out reusable-workflow co-located files with repository: ${{ job.workflow_repository }} and ref: ${{ job.workflow_sha }}; #704 uses that native, immutable identity directly while also delivering the hourly cadence and bounded retry contract. Keeping a second parser-based implementation with a legacy canonical_ref input would duplicate the same security fix and increase workflow-contract maintenance.