feat: Circuit breaker for failing domains

[Snider]

Summary

Stop hammering domains that are consistently failing.

Use Case

If a domain returns 5 errors in a row, stop trying for a while instead of wasting time and bandwidth.

Circuit States

CLOSED → OPEN → HALF-OPEN → CLOSED
  │        │        │
  │        │        └─ Test request succeeds
  │        └─ After cooldown period
  └─ Failure threshold exceeded

Behaviour

• CLOSED: Normal operation, requests flow through
• OPEN: All requests fail immediately, no network calls
• HALF-OPEN: Allow one test request to check recovery

Commands

# Enable circuit breaker (default)
borg collect website https://example.com

# Disable
borg collect website https://example.com --no-circuit-breaker

# Custom thresholds
borg collect website https://example.com \
  --circuit-failures 3 \
  --circuit-cooldown 60s

Configuration

circuit_breaker:
  failure_threshold: 5      # failures before opening
  success_threshold: 2      # successes to close
  cooldown: 30s             # time in open state
  half_open_requests: 1     # test requests in half-open

Logging

[WARN] Circuit OPEN for api.github.com (5 consecutive failures)
[INFO] Circuit HALF-OPEN for api.github.com (testing recovery)
[INFO] Circuit CLOSED for api.github.com (recovered)

Acceptance Criteria

• Three-state circuit breaker
• Per-domain tracking
• Configurable thresholds
• Automatic recovery testing
• Clear logging of state changes

[google-labs-jules]

Jules is on it. When finished, you will see another comment and be able to review a PR.

[google-labs-jules]

Ready for a review! A PR has been created.