feat(review): add AMS live-gate-thresholds read endpoint

[RealDiligent]

Summary

• Root cause / need: AMS needs a field-limited read of ORB live (or soaking shadow) gate thresholds without audit metadata (feat(review): read-only endpoint exposing ORB's live self-tuned gate thresholds #6486).
• Fix: GET /v1/repos/:owner/:repo/live-gate-thresholds returns snake_case confidence_floor / scope_cap_* from authoritativeGateOverride + toLiveGateThresholdFields, gated like gate-config/effective; 404 when none active.
• Impact: AMS can probe live thresholds safely.

Closes #6486

Supersedes closed #6545 (codecov/patch).

Test plan

• Unit projection edge cases (live/shadow/null/scopeCap-only/confidenceFloor-only/empty)
• Integration route coverage
• CI + codecov/patch ≥ 99%

Risk / tradeoffs

• Read-only; no behavior change to auto-apply write path.

[RealDiligent]

Opened from a polluted branch; superseded by clean follow-ups.

[superagent-security]

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.60%. Comparing base (c9984c1) to head (062604e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6551   +/-   ##
=======================================
  Coverage   93.60%   93.60%           
=======================================
  Files         675      675           
  Lines       67742    67764   +22     
  Branches    18618    18627    +9     
=======================================
+ Hits        63409    63431   +22     
  Misses       3360     3360           
  Partials      973      973           

Flag	Coverage Δ
shard-1	43.86% <59.09%> (-0.18%)	⬇️
shard-2	37.07% <9.09%> (+0.26%)	⬆️
shard-3	32.47% <18.18%> (+0.04%)	⬆️
shard-4	34.30% <18.18%> (-0.61%)	⬇️
shard-5	31.52% <9.09%> (+0.39%)	⬆️
shard-6	45.69% <81.81%> (+0.08%)	⬆️

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Files with missing lines	Coverage Δ
src/api/routes.ts	94.53% <100.00%> (+0.02%)	⬆️
src/openapi/schemas.ts	100.00% <100.00%> (ø)
src/openapi/spec.ts	99.43% <100.00%> (+<0.01%)	⬆️
src/review/auto-apply.ts	99.22% <100.00%> (+0.05%)	⬆️