API-mode usage reader from response headers (built, inactive)

[Ryan-Atkinson87]

Implement the API-key-mode usage reader against the UsageReader interface (#30). Built for
completeness; inactive until an API key is configured (the human is subscription-only for now).

Spec: §6.2 (Reading usage — API mode), §6.1 (Meters).

Acceptance criteria

• Reads rate-limit headers (anthropic-ratelimit-tokens-remaining, anthropic-ratelimit-tokens-reset, and siblings) per call via the SDK's raw-response accessor — no polling endpoint.
• Maps headers onto the Usage meter model, governing-meter selection, and reader interface #30 meters (api_budget and any token/request windows the headers expose).
• Selected only when provider.plan: api; otherwise dormant. The subscription path (Subscription usage reader: OAuth /api/oauth/usage poll with degradation chain #31) is unaffected.
• Reading stays behind the Claude adapter boundary (Spec §3.3); the engine talks to UsageReader.
• Both success and failure paths explicit; a missing/un-parseable header degrades rather than crashes.
• Unit tests cover header parsing → meters using fixture responses (path is not exercised live).

Depends on: #30
Depends on: #17