feat(bg): B-0442.2 — merged-PR fetch via gh CLI (10 tests pass)

[AceHack]

Summary

Slice 2 of B-0442. The missed-substrate cascade detector now fetches recent merged PRs via `gh pr list --state merged --search "merged:>{iso}"` and reports the candidate set within a configurable lookback window (default 30min).

What lands

• spawnSync + sonarjs lint suppression with rationale
• Adapter pattern for deterministic tests
• gh JSON output type-guarded
• New `--lookback-min` flag (default 30)
• `cascadesDetected` still 0 in slice 2 — slice 3 will fetch branch HEADs + compare against squash content

Tests

```
bun test
10 pass
0 fail
20 expect() calls
```

Three-service mechanization status

Service	Skeleton	Real detection
B-0440 standing-by	✅ #3006	✅ #3011 (commit-history poll)
B-0441 backlog-ready	✅ #3007	✅ #3012 (backlog row scan)
B-0442 missed-substrate	✅ #3008	THIS PR (merged-PR fetch)

🤖 Generated with Claude Code

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 6229a2c836

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[Copilot]

Pull request overview

This PR advances the B-0442 missed-substrate detector from a slice-1 skeleton to slice 2 by adding a gh pr list --state merged-based fetch of recently merged PRs and reporting the size of the candidate window via the detector’s JSON output.

Changes:

• Add gh CLI integration (via spawnSync) to fetch merged PRs within a configurable lookback window.
• Extend config/CLI parsing with --lookback-min (default 30) and include candidatesScanned in the poll result.
• Update tests to use injected adapters for deterministic pollOnce behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tools/bg/missed-substrate-detector.ts	Implements merged-PR fetching via gh, adds lookback config/flag, and expands poll result shape.
tools/bg/missed-substrate-detector.test.ts	Updates tests for slice 2, exercising adapter-injected pollOnce and new CLI flag parsing.

Comments suppressed due to low confidence (2)

tools/bg/missed-substrate-detector.ts:75

• The gh call drops stderr and on any failure (non-zero exit, missing stdout, JSON parse/type-guard failure) returns an empty list. That makes pollOnce report "no merged PRs" even when gh is missing/auth-failed/output-changed. Please surface fetch failures distinctly (e.g., return an error from the adapter and reflect it in note, and/or capture stderr) so operational issues aren’t misreported as an empty candidate set.

      { encoding: "utf8", stdio: ["ignore", "pipe", "ignore"] },
    );
    if (result.status !== 0 || !result.stdout) return [];
    try {

tools/bg/missed-substrate-detector.ts:110

• note treats merged.length === 0 as "no merged PRs", but merged can also be empty due to gh failures/parse errors. Once fetch failures are surfaced, consider distinguishing these cases in the note so the JSON output is diagnostic in cron/daemon logs.

    note: merged.length === 0
      ? `no merged PRs in last ${config.lookbackMin}min lookback window`
      : `${merged.length} merged PR(s) in last ${config.lookbackMin}min; slice 3 will compare each branch HEAD against squash content`,
  };