feat(n8-c1): SSOT-existence arm on Prior-art trailers (pre-push)

[artyhoo]

What

Wave N8 A-phase item C1 — the highest correctness-ROI deterministic-offload candidate per findings §7 D3 / wave-sequencing-plan §5.3 D3.

Mechanises the §1.9 "SSOT citation existence-check" prose probe (phase-research-coverage.md §1.9). A capability commit's Prior-art: prior-art-evaluations.md#N trailer is now verified that #N resolves to a real register row — not merely that the trailer is present. This catches the Wave-7 M2 incident class (a trailer cited a non-existent #N and passed by presence alone).

Channel (per rule-enforcement-channel-selection)

Detectable → gate at the earliest reachable channel = pre-push, where commit trailers exist. Principle 08 already covers in-file prior-art-evaluations.md#N citations in research files; it cannot see commit-message trailers. C1 covers exactly that complementary surface — two independent channels over one register.

Changes (all under packages/core/hooks/)

• checks/prior-art.ts — extractCitedSsotIds() + loadSsotIds(); checkTrailerBody gains an optional ssotIds set → new code 3 (broken citation); runPriorArtCheck routes it into report.brokenCitations. Pure/injected — ssotIds undefined ⇒ graceful no-op (unreadable register never blocks a push; isolated test repos hit this path).
• pre-push.ts — priorArtSection reads the register, passes the id-set, blocks (exit 1) on broken citations with a grep-to-verify hint.
• prior-art.test.ts — +17 tests: paired-negative (#1 passes / #999 blocks), multi-missing report, free-form-trailer pass, escape-hatch precedence, pre-cutoff bypass, arm-disabled-without-ssotIds, loader/extractor units.

Verification

• prior-art.test.ts — 65/65 pass (48 prior + 17 new).
• npm run test:principles — 106/106; tsc --noEmit clean.
• tests/hooks/prior-art-trailer-hook.test.sh — 8/8 (graceful-degradation path confirmed).
• pre-push gate green on push.

§1.7

• Forward-check: complies with no-paid-llm-in-ci (deterministic grep, zero API calls); build-first-reuse-default (extends own prior-art.ts §7 check — no upstream analog for "grep our own register"); doc-authority-hierarchy (no rule/doc edited).
• Backward-check: scope is the commit-trailer surface only; principle 08's in-file broken-ref check is unchanged and uncoupled.

Observations (NOT fixed here — out of C1 scope)

1. Stale local test, CI-invisible: packages/core/hooks/pre-push.test.ts:59 asserts the orchestrator invokes audit-self/audit-ai-docs.test.sh, but Wave 10.4 migrated that to …/audit-ai-docs.test.ts. The assertion fails locally on clean staging (pre-dates this PR); CI is green because audit-self.yml runs only test:principles + the specific audit-ai-docs.test.ts, not the hooks/ vitest suite. Want me to fix the stale assertion as a separate task?
2. §1.9 prose now stale: phase-research-coverage.md §1.9 says the existence probe is "prose-only today" — now mechanised. .claude/rules/* is maintainer-owned (Artifact Ownership Contract), so I left it. Suggest a one-line maintainer edit noting C1 wired it at pre-push.