feat(boatstack): observe the published PR's phase — checks, review, mergeability

[bigboateng]

First slice of the published → merged program: before Boatstack can prescribe anything past publish-pr, it needs eyes on where the published PR actually stands. This PR adds exactly that observation — and nothing else. No prescription, actor, or goal behavior changes.

What changed

• observePublishedPR widens its single read-only gh pr view call with statusCheckRollup, reviewDecision, mergeStateStatus, mergeable, baseRefName. If an older gh rejects the enriched field list, it falls back to the legacy list so the basic lifecycle observation is never lost.
• New pr_phase.go: a pure, total, fail-closed derivation to PRPhase — PR_CHECKS_PENDING, PR_CHECKS_FAILING, PR_CHANGES_REQUESTED, PR_REVIEW_REQUIRED, PR_MERGE_ELIGIBLE, PR_MERGED, PR_CLOSED, and PR_UNKNOWN for anything not understood with certainty (conflicts, stale base, drafts, novel check vocabularies all stay Unknown → operator).
• Both statusCheckRollup shapes (CheckRun and StatusContext) are classified; failing check names surface (capped at 8) so a status reply can say which job is red.
• Additive NextStatus/RecoveryStatus fields (pr_phase, pr_review_decision, pr_merge_state, pr_failing_checks; schema versions unchanged) and phase-aware published reasons, with the pre-phase sentences kept as the degraded fallback.

Boundary conformance

• Boundary: external GitHub state entering read-only resolution output.
• Control law: pr-phase-derives-only-from-live-observation — the phase derives only from one live observation at resolution time; never persisted, never accepted from text; underivable → PR_UNKNOWN.
• Authorized actor: the read-only resolvers (next-status, recovery-status) observing via gh.
• Required evidence: conformance classes in pr_phase_conformance_test.go — positive (each understood combination → its phase through the real ResolveNext path), negative (degraded/malformed/unrecognized → PR_UNKNOWN), bypass (non-terminal observation leaves the delivery ledger byte-identical; terminal cache write unchanged), failure-state (older gh → legacy field fallback).
• Failure behavior: fail-closed to PR_UNKNOWN; gh unavailability degrades to exactly the pre-change behavior.
• Release condition: all existing conformance suites green; persistObservedTerminalPRState byte-for-byte behavior preserved.

Evidence

• go build ./... && go vet ./... && go test ./... — pass (full package suite).
• python3 -m unittest tests.test_product_loop — 18/18 pass.
• Release note added: 2026-07-28-pr-phase-observation.md; release_notes.py preflight — PASS.

Next slices: flow frontier dashboard, flow watch, then the configurable delivery.terminal: merged goal.