chore(review): deprecate gateCheckMode as a computed read-back value (#4618)

[JSONbored]

Summary

Part of #4618's item 2 (config-surface-reduction epic). gateCheckMode was a legacy shadow of reviewCheckMode (the actual check-run publish authority), kept in sync only by dual-write logic scattered across the DB layer, both settings-write API schemas, and the yml gate: resolver.

• gateCheckMode is dropped from every write schema (maintainer dashboard PUT, internal full-replace POST) and is no longer an accepted upsertRepositorySettings input.
• On every read (DB row mapping, yml gate: block resolution), gateCheckMode is now always recomputed from reviewCheckMode — "enabled" when reviewCheckMode !== "disabled", else "off".
• The yml settings.gateCheckMode key still parses for back-compat with existing .gittensory.yml files, but now derives reviewCheckMode from it (the historical effect), rather than the old reverse direction.
• Fixes a related latent bug: the yml gate: block only re-derived gateCheckMode when gate.enabled was set, so a manifest setting ONLY gate.checkMode could leave the back-compat display field silently stale (showing "off" while the check was actually publishing, or vice versa).
• The maintainer dashboard's "Review agent check" toggle now writes reviewCheckMode directly instead of the deprecated field (same off/enabled UX, real field underneath).
• Refreshed README/CONTRIBUTING/docs references that described gateCheckMode as the "master switch".

No DB migration: the gate_check_mode column is left in place (still written, now always self-consistent with reviewCheckMode) since nothing reads it independently anymore.

Test plan

• npx tsc --noEmit (root + apps/gittensory-ui) — clean
• Full unit + integration suite: 685 files / 13,635 tests passing
• npm run test:coverage (unsharded) — zero uncovered statements/branches in any changed line range across src/db/repositories.ts, src/api/routes.ts, src/services/maintainer-activation.ts, src/signals/focus-manifest.ts
• npm run engine-parity:drift-check, npm run ui:openapi:check, npm run ui:openapi:settings-parity, npm run docs:drift-check, npm run manifest:drift-check — all pass
• npm run ui:lint (0 errors) + npm audit --audit-level=moderate (0 vulnerabilities)

[superagent-security]

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

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	gittensory-ui	2a5d82b	Commit Preview URL Branch Preview URL	Jul 10 2026, 10:22 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.21%. Comparing base (12147aa) to head (2a5d82b).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4749      +/-   ##
==========================================
- Coverage   94.22%   94.21%   -0.01%     
==========================================
  Files         439      439              
  Lines       38704    38702       -2     
  Branches    14100    14098       -2     
==========================================
- Hits        36468    36464       -4     
- Misses       1576     1577       +1     
- Partials      660      661       +1     

Files with missing lines	Coverage Δ
packages/gittensory-engine/src/focus-manifest.ts	99.11% <100.00%> (+<0.01%)	⬆️
src/api/routes.ts	94.25% <ø> (-0.01%)	⬇️
src/db/repositories.ts	96.72% <100.00%> (ø)
src/services/maintainer-activation.ts	100.00% <ø> (ø)
src/signals/focus-manifest.ts	99.64% <100.00%> (ø)
src/types.ts	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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