Add shared/global contributor blacklist (#1425)

[tmimmanuel]

Summary

• Added shared/global contributor blacklist persistence via new migration-backed singleton table global_contributor_blacklist and DB read/write APIs.
• Wired global blacklist into repository settings resolution so effective settings are .gittensory.yml/DB + shared/global list.
• Added focused unit coverage for DB round-trip, resolution merge behavior, and a self-tune path regression guard.

Validation

• npm run db:migrations:check
• npm run typecheck
• npm test -- test/unit/contributor-blacklist.test.ts test/unit/focus-manifest.test.ts test/unit/selftune-readback.test.ts
• npm run test:unit

Closes #1425

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.47%. Comparing base (7c7a37e) to head (aac6d5a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1531   +/-   ##
=======================================
  Coverage   95.47%   95.47%           
=======================================
  Files         196      196           
  Lines       21197    21208   +11     
  Branches     7662     7666    +4     
=======================================
+ Hits        20237    20248   +11     
  Misses        383      383           
  Partials      577      577           

Files with missing lines	Coverage Δ
src/db/repositories.ts	96.14% <100.00%> (+0.02%)	⬆️
src/settings/repository-settings.ts	100.00% <100.00%> (ø)
src/signals/focus-manifest.ts	98.14% <100.00%> (+<0.01%)	⬆️

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[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.