feat: GitHub REST API combined-status check for multi-suite CI aggregation

[mdproctor]

Context

devtown#86 implements webhook-only CI aggregation with pessimistic sticky-failure policy. This has two known limitations:

1. Aggregation gap: We don't know the total expected suite count without the REST API. A 'passing' status may be premature if additional workflows have not reported.
2. Atomicity race: Two concurrent check_suite webhooks can both read zero existing suites and both signal 'passing.'

What to do

Query the GitHub Checks API combined status for the HEAD SHA before signaling ci.status = 'passing'. This eliminates both the aggregation gap and the atomicity race.

Requires: GitHub REST client (REST client dependency, GitHub App or PAT auth).

Refs #86

[mdproctor]

Design Spec — GitHub Combined-Status Check (rev 3)

$(cat /Users/mdproctor/claude/public/casehub/devtown/specs/2026-06-23-github-combined-status-design.md)