Add error handling in "Get linked issue body" step to distinguish between "no linked issue" and API failures

What

In .github/workflows/_ai-pr-review.yml, the "Get linked issue body" step currently uses || true to swallow all errors from the pipeline:

ISSUE_NUM=$(gh pr view ${{ github.event.pull_request.number }} --json body,title \
  | jq -r '[.title, .body] | join(" ")' \
  | python3 -m cicaid_devtools.lib.linked_issue || true)

This needs to be refactored to distinguish between:

1. Valid outcome: No linked issue found (the python3 -m cicaid_devtools.lib.linked_issue command exits with a specific code or output indicating no linked issue)
2. Error: gh pr view or jq fails (e.g., authentication failure, network issue, malformed response)

Why

The current implementation masks real failures. If gh pr view fails due to a token issue or API error, ISSUE_NUM will be empty and the step will proceed as if "No linked issue found" — hiding an authentication or infrastructure problem. This makes debugging difficult and could lead to incorrect workflow behavior (e.g., skipping a review that should have happened, or proceeding without context that should have been available).

How

1. Separate the commands: Split the pipeline into distinct steps or use explicit error handling:
   • First, fetch the PR data with gh pr view and check its exit code
   • Then, run the python3 -m cicaid_devtools.lib.linked_issue command separately
2. Check exit codes explicitly: Instead of || true, capture exit codes and handle them:
   • If gh pr view fails, fail the step with a clear error message
   • If python3 -m cicaid_devtools.lib.linked_issue exits with a "no linked issue" code, set ISSUE_NUM to empty and continue
   • If python3 -m cicaid_devtools.lib.linked_issue exits with an unexpected error, fail the step
3. Add error output: Use GITHUB_OUTPUT or echo "::error::" to provide clear diagnostics when failures occur

Files Affected

• .github/workflows/_ai-pr-review.yml

Constraints

• Must not break the existing behavior when no linked issue is found (the workflow should continue normally)
• Must not introduce new dependencies or require changes to the cicaid_devtools package
• Must maintain compatibility with the existing continue-on-error pattern used elsewhere in the workflow
• Out of scope: fixing the misleading condition in the "Fail if provider did not approve" step (separate issue)

LLM tier

Sonnet — This requires moderate design judgment to properly handle exit codes and error paths without breaking existing behavior. It's a single-file change but involves non-trivial logic restructuring.

Success looks like

• When gh pr view fails (e.g., invalid token), the step fails with a clear error message indicating the API call failed
• When no linked issue exists, the step succeeds with ISSUE_NUM empty and the workflow continues as before
• When python3 -m cicaid_devtools.lib.linked_issue encounters an unexpected error, the step fails with diagnostic information
• The workflow behaves identically to before for all valid scenarios (linked issue found, no linked issue)

Failure looks like

• The step still swallows errors from gh pr view or jq
• The workflow fails when no linked issue is found (regression)
• Error messages are unclear or don't indicate which command failed
• The change introduces shell syntax errors or breaks the existing pipeline structure

Follow-up from AI review of PR #3.

Implementation notes

FILES: .github/workflows/_ai-pr-review.yml
DONE: Refactor the "Get linked issue body" step to explicitly check exit codes from gh pr view and the python3 -m cicaid_devtools.lib.linked_issue command, failing the step on API/command errors while preserving the current behavior of continuing with an empty ISSUE_NUM when no linked issue exists.