feat(miner-governor): self-plagiarism throttle across the miner's own repos

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An autonomous miner working many repos in parallel risks submitting near-duplicate fixes/PRs derived from the same underlying diff across multiple targets (self-plagiarism, distinct from copying someone else's work) — a distinct slop-at-scale vector the roadmap flags. This issue adds a Governor-consulted check that compares a prospective PR's diff fingerprint against the miner's own RECENT submission history and throttles (denies or demotes) near-duplicate submissions across repos, reusing the deterministic election logic already proven for duplicate-issue-cluster adjudication.

This gates a PR submission based on the SAME miner's own recent history only — a self-referential, fail-closed throttle (see the required tests below), not a shared or cross-actor safety chokepoint. Open, scored contributor work, provided the fail-closed behavior on ambiguous data is thoroughly tested.

Deliverables

• A selfPlagiarismCheck(candidateFingerprint, recentOwnSubmissions) function consulted by the Governor chokepoint before an open_pr action is allowed
• Reuses the same claim-time/earliest-wins election style as isDuplicateClusterWinnerByClaim (fail closed on missing/ambiguous timing data) rather than inventing new adjudication logic
• A configurable similarity threshold (not hard-coded) sourced from .gittensory-miner.yml, with a conservative built-in default
• Tests: a genuinely distinct PR passes, a near-duplicate diff across two different target repos is throttled, and missing/ambiguous fingerprint data fails closed (denies rather than assumes uniqueness)
• Denials recorded to the governor ledger with the flagged prior submission referenced

References

• src/signals/duplicate-winner.ts (isDuplicateClusterWinnerByClaim, fail-closed-on-sparse-data pattern) — explicitly reused here for self-claim adjudication, per the architecture skeleton
• src/signals/slop.ts (buildDuplicateClusterFinding) — the existing duplicate-cluster signal vocabulary this throttle's rationale should stay consistent with
• The Governor chokepoint (companion maintainer issue) — this check gates the open_pr write path specifically

[linear-code]

JSO-2442