compiler: auto-derive and warn on missing path: for cross-repo checkout entries

[Copilot]

When a checkout: block has a cross-repo entry without an explicit path:, the compiler was emitting GH_AW_CHECKOUT_PATH_N: "", causing safe-outputs handlers (e.g. push_to_pull_request_branch) to fail at runtime with "Repository not found in workspace."

Changes

• pkg/workflow/checkout_path_validation.go — new validateCrossRepoCheckoutPaths validator that:

  • Auto-derives path: from the repo-name segment of owner/repo (e.g. githubnext/gh-aw-side-repo → gh-aw-side-repo) and mutates the CheckoutConfig before YAML generation, ensuring both the emitted actions/checkout step and GH_AW_CHECKOUT_PATH_N in the manifest are non-empty
  • Emits a deprecation warning directing authors to add an explicit path: to silence it
  • Skips dynamic ${{…}} expressions — cannot be resolved at compile time; common patterns like repository: ${{ github.repository }} (trusted pull_request_target checkout) must not warn

• pkg/workflow/compiler_validators.go — wires validateCrossRepoCheckoutPaths into validateCoreToolConfiguration, before checkout step generation

# Before: missing path triggers empty GH_AW_CHECKOUT_PATH_0 → runtime failure
checkout:
  - repository: githubnext/gh-aw-side-repo
    token: ${{ secrets.TEMP_USER_PAT }}
    fetch: ["*"]
    fetch-depth: 0

# After: compiler auto-derives path and emits deprecation warning;
# explicit path: silences the warning
checkout:
  - repository: githubnext/gh-aw-side-repo
    path: gh-aw-side-repo   # ← add this
    token: ${{ secrets.TEMP_USER_PAT }}
    fetch: ["*"]
    fetch-depth: 0

[github-actions]

Hey @dsyme 👋 — nice work scoping out the validateCrossRepoCheckoutPaths fix for cross-repo checkout entries! The task breakdown in the body is clear and the change is tightly focused on one compiler concern.

One thing to keep in mind before this moves out of WIP:

• Ship tests with the implementation — the task list already calls this out ("Add unit tests for the new validation function"), so just make sure they land in the same PR. The new validation path (deprecation warning + auto-derive path:) has at least three meaningful cases to cover: entry with no explicit path (auto-derive fires), entry with an explicit path (no-op), and a non-cross-repo entry (should be untouched).

If you'd like a hand getting this across the finish line, here's a ready-to-use prompt:

Implement the following in the gh-aw compiler:

1. Add a `validateCrossRepoCheckoutPaths` function in `compiler_validators.go` that:
   - Iterates over all checkout entries in the workflow
   - For cross-repo entries (repo field references a different repo) that have no explicit `path:` set, auto-derives a `path:` value and emits a deprecation warning
   - Leaves entries with an explicit `path:` unchanged
   - Leaves non-cross-repo entries unchanged

2. Wire `validateCrossRepoCheckoutPaths` into `validateCoreToolConfiguration` so it runs as part of normal compilation.

3. Add unit tests in the corresponding `_test.go` file covering:
   - Cross-repo entry with no explicit path → path auto-derived, warning emitted
   - Cross-repo entry with explicit path → no change, no warning
   - Same-repo entry → no change, no warning

4. Run the full test suite and confirm all existing tests still pass.

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[github-actions]

🔍 PR Triage — §28395315609

Field	Value
Category	bug
Risk	medium
Score	47 / 100
Action	batch_review
Batch	—

Score breakdown: impact 25 + urgency 10 + quality 12

Compiler fix: auto-derives path: for cross-repo checkout entries missing it, fixing runtime "Repository not found in workspace" failures. 3 files (+292/-0). Draft, no CI yet. Correctly skips dynamic ${{...}} expressions. Promote from draft when ready.

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