pr-merge-autopilot.sh consistently reports "Failed to merge" when merge actually succeeded #173

Context

Over the course of 2026-05-12/13, every successful auto-merge via scripts/pr-merge-autopilot.sh printed:

```
[HH:MM:SS] Conditions met for merge
✗ Failed to merge PR #NNN
```

…immediately followed by GitHub showing the PR as merged with a real merge commit (PRs #157, #158, #159, #161, #162, #163, #164, #166 all hit this).

The autopilot's gh pr merge call seems to race against GitHub's auto-merge feature — by the time the script invokes gh pr merge --squash, the PR has already auto-merged (because all required checks went green during the polling loop), and the merge call returns non-zero because there's nothing left to merge.

This forced a manual gh pr view <n> after every autopilot run to confirm actual merge status, which defeats the autopilot's purpose.

Proposed solution

After the gh pr merge call returns non-zero, the script should:

1. Re-query gh pr view <PR> --json state,mergedAt,mergeCommit
2. If state === "MERGED" (regardless of merge-call exit code) → report success
3. Only report failure if the PR is still OPEN after the merge attempt

The current behavior trusts the gh CLI's exit code as authoritative; the fix is to trust the GitHub server state instead.

Acceptance criteria

• Race-merged PRs (auto-merged before script's merge call lands) report success, not failure
• Genuinely-failed merges (e.g., still BLOCKED) continue to report failure with a clear reason
• Exit code matches the actual server state