refactor(reliability): collapse 3x retry + 3x circuit-breaker to one canonical each

[kcenon]

What

There are three parallel retry implementations and three hand-rolled circuit breakers; the "canonical" ADR-0004 machinery is unused. Collapse each set to one canonical implementation behind a behavior-parity test matrix.

Why

Triplicated resilience logic means fixes and tuning must be applied in three places and silently diverge. Consolidating reduces maintenance/audit surface and makes the ADR-0004 strategy actually load-bearing.

Where (analysis-located, LOC re-measured)

• Retry (~2,210 LOC across 4 files): src/error-handler/RetryHandler.ts (642), src/error-handler/RetryExecutor.ts (615), src/worker/RetryHandler.ts (555), src/error-handler/RetryMetrics.ts (398).
• Circuit breakers (~867 LOC across 2 files + 1 embedded): src/error-handler/CircuitBreaker.ts (517), src/pr-reviewer/CICircuitBreaker.ts (350), plus the breaker embedded in analysis-orchestrator.

How

1. Pick/define the canonical retry engine and the canonical circuit breaker.
2. Write a behavior-parity test matrix BEFORE deleting any duplicate (retry: backoff sequence, jitter bounds, max-attempts, give-up signal; breaker: open/half-open/close thresholds, reset-on-success).
3. Migrate every caller, then delete the duplicates.

Acceptance

• The behavior-parity matrix passes against the canonical implementation before duplicates are removed.
• Full suite green after migration; duplicate files deleted; all callers point at the canonical module.

Part of #866.