compiler: require (or auto-generate) `path:` for cross-repo `checkout:` entries

[dsyme]

Problem

When a workflow's checkout: block contains a cross-repository entry (i.e., repository: differs from the host repo) without an explicit path::

# Source .md — missing path:
checkout:
  - repository: githubnext/gh-aw-side-repo
    token: ${{ secrets.TEMP_USER_PAT || secrets.GH_AW_TEST_PAT }}
    fetch: ["*"]
    fetch-depth: 0
    # ← no path: declared

the compiler emits the checkout manifest env var as empty:

GH_AW_CHECKOUT_PATH_0: ""   # ← empty; push handler cannot locate the repo on disk

Any safe-output handler that needs to find the side repo on disk (e.g. push_to_pull_request_branch) reads this manifest, gets an empty path, and fails:

Repository 'githubnext/gh-aw-side-repo' not found in workspace.
Check out the target repo with actions/checkout and set its 'path' input
so the checkout can be located. If checking out multiple repositories,
ensure each actions/checkout step uses the appropriate 'path' input.

This is the active blocker for test-copilot-siderepo-push-to-pull-request-branch-using-dispatch and test-copilot-siderepo-sparse-push-to-pull-request-branch-using-dispatch (tracked in #41292, runs 28349548237 and 28349741075, gh-aw commit d6d91600fd9a).

Proposed fix

Option A — auto-derive path:: When the checkout: entry has no explicit path:, the compiler derives one automatically (e.g. the repo-name portion of owner/repo, so githubnext/gh-aw-side-repo → path: gh-aw-side-repo). This is the least-friction option and mirrors what actions/checkout itself recommends when checking out multiple repos.

Option B — require path: at compile time: If no path: is declared on a cross-repo checkout entry, the compiler emits a validation error/warning instructing the author to add one. This is stricter but makes the intent explicit.

Either way, GH_AW_CHECKOUT_PATH_N in the checkout manifest must be set to the resolved path.

Workaround (applied in githubnext/gh-aw-test)

Adding path: gh-aw-side-repo to the checkout: block in the source .md file fixes the issue:

# Source .md — with path: added
checkout:
  - repository: githubnext/gh-aw-side-repo
    token: ${{ secrets.TEMP_USER_PAT || secrets.GH_AW_TEST_PAT }}
    path: gh-aw-side-repo        # ← added
    fetch: ["*"]
    fetch-depth: 0

After recompiling, the compiler correctly emits GH_AW_CHECKOUT_PATH_0: "gh-aw-side-repo" and the push handler resolves the repo on disk.

[dsyme]

Option B — require path: at compile time: If no path: is declared on a cross-repo checkout entry, the compiler emits a validation error/warning instructing the author to add one. This is stricter but makes the intent explicit.

We should initially give a warning that omitting path: is deprecated suggest the path to be added.