feat: add optional --summaries semantic layer for PR deltas

[diegomarino]

Why

A downstream consumer (batutas) runs a fail-closed merge gate needing semantic facts — ciRollup === 'green', mergeable, unresolvedThreads === 0. The v0.2 per-delta to fingerprint can't supply them: ci/review/reviews are opaque change-detection digests (--detail marks them opaque: true). Correct for "did CI change?", useless for "is CI green?" — forcing consumers into a second GitHub fetch.

What

--summaries attaches an optional, normalized delta.summary to every PR delta with an observed to state, derived purely from the same observation that produced the fingerprints (no second fetch):

"summary": {
  "ciRollup": "green|failed|pending|none",   // zero checks ⇒ none, never green
  "reviewDecision": "approved|changes_requested|review_required|none",
  "mergeable": "mergeable|conflicting|unknown",  // honest tri-state, not a boolean
  "state": "open|closed|merged",
  "isDraft": false,
  "unresolvedReviewThreads": 0,
  "headSha": "<40-hex>"
}

Design invariants

• Sibling delta.summary, never nested in to. deltaIdentity() hashes only to/from/classes/missingTicks, so the content-addressed delta.id and the whole report body stay byte-identical whether or not the flag is set. Existing consumers see zero diff.
• Zero checks ⇒ ciRollup: 'none', never 'green' — fail-closed gates decide what "no CI" means. Precedence: failed > pending > green. Classification keys on both status and conclusion, so an in-progress {status:'IN_PROGRESS', conclusion:''} CheckRun is pending, not green.
• Honest enums, not booleans. mergeable:'unknown' / reviewDecision:'none' preserve GitHub's tri-state / not-yet-computed truth (merged PRs legitimately report unknown).
• schemaVersion stays 1 per the repo's additive-never-bumps policy. Only --help-json grows (deltaSummaryFields/deltaSummaryEnums).
• Mirrored onto the outpost webhook payload under the same flag.

Testing

TDD against real captured GraphQL payloads (test/fixtures/summaries/, recorded from live PRs on 2026-07-11) run through the real fetchPRs → normalizePr → prFingerprint → prSummary path, covering none (nodejs/node#64097), green (this repo #12), pending (nodejs/node#63949, real in-progress CheckRun), failed (react/react#36863). Plus CLI acceptance (none→green ci-changed, additive byte-stability) and --help-json schema tests. 211 tests pass; lint + prettier clean.

Acceptance

• No-checks PR ⇒ ciRollup: 'none'; a green status ⇒ ci-changed delta with summary.ciRollup: 'green'
• Existing consumers (no flag) see zero diff (delta.id + all fields byte-identical)
• README + docs/contract.md document the schema precisely enough to build a Zod/JSON-Schema validator (also machine-readable via gh-delta/contract and --help-json)

Validated with a Codex blind-debate peer review (classification table checked against live GitHub enums; three scope gaps — outpost propagation, DELTA_FIELDS exhaustiveness test, in-progress fixture coverage — folded in).

Version bump is release-please-owned: this feat: will drive 0.2.0 → 0.3.0.