feat: ownership resolver chain and issue deduplication

[shouze]

Summary

Pluggable resolver chain for automatic assignee detection, and deduplication helper to avoid creating duplicate dispatch issues.

Tasks

• src/core/ownership.ts
  • Resolver chain: teamsResolver → codeownersResolver → mappingResolver → fallbackResolver
  • resolveOwners(ctx, chain?) — first non-null result wins
  • External mapping: centralRepo/.github-issue-ops/owners.json
• src/core/dedup.ts
  • findExistingDispatchIssue(token, owner, repo, epicUrl) — search query: repo:… label:github-issue-ops in:body {epicUrl} is:open
  • buildDedupMap(token, org, repos, epicUrl) — batch lookup

Resolver priority

1. GitHub org teams matching configured teamPrefixes
2. CODEOWNERS catch-all rules (* or /)
3. JSON mapping in central repo
4. Fallback → empty []

Acceptance criteria

• Unit tests in src/core/ownership.test.ts and src/core/dedup.test.ts pass
• Resolver chain stops at the first non-null result
• Dedup returns null when no issue matches