Complete scalable MCP contracts and PR health coverage

[morluto]

Closes #20.

Summary

• add typed, bounded GitHub PR-health reads for checks, review threads, merge state, merge queue, closing issues, and changed files with independent completeness and retry guidance
• add stale-safe atomic facet CAS, opt-in issue timelines, exact resolution provenance, offline portfolio overlap, and explicit PR-to-opportunity/workspace links
• vectorize coverage reads and durable-job cancellation, and expose structured phase/item progress instead of MCP-visible free-form progress
• update MCP routing, schemas, catalog budgets, workflow docs, and real stdio coverage

Contract and safety notes

• corpus and portfolio reads remain offline
• GitHub SDK and GraphQL DTOs stay inside internal/github
• complete child snapshots replace atomically; concurrent equal-clock refreshes use coverage-sequence CAS
• missing mergeability, health, or overlap coverage remains unknown/retryable, never pass/no-overlap
• closing-issue references remain relationship evidence and are not mislabeled as completed resolutions
• no GitHub mutation or repository-controlled execution was added

Research

The implementation was checked against GitHub REST/GraphQL pagination and rate-limit guidance, MCP progress/task guidance, DeepWiki architecture analysis, and Gitcrawl's pagination/snapshot patterns. Gitcrawl compatibility and imports were intentionally not carried over.

Verification

• gofmt on changed Go files
• go test ./...
• real subprocess MCP stdio flows
• catalog schema, routing, size, partial-result, cancellation, rate-limit, concurrency, provenance, and migration regression tests
• autoreview: clean, no accepted/actionable findings

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