feat(devtools/claude-usage): Anthropic OAuth usage API client + mapping

[jaeyeom]

Parent

Part of #199 — port claude-usage into devtools/.

Depends on credential resolution (#199 child: OAuth credentials issue).

Problem

Need a small client for Anthropic’s OAuth usage endpoint that powers the 5-hour and 7-day windows, without importing claude-hud’s Node module.

API notes (as used by claude-hud)

• Endpoint: GET https://api.anthropic.com/api/oauth/usage (confirm path/headers against current Claude Code behavior while implementing)
• Auth: OAuth bearer token from Claude Code credentials
• User-Agent used by claude-hud: claude-code/2.1 (match or document divergence)
• Response includes roughly:
  • five_hour.utilization, five_hour.resets_at
  • seven_day.utilization, seven_day.resets_at
• Map utilization → percent fields fiveHour / sevenDay
• Plan name from subscription type / fallback (claude-hud has getUsagePlanNameFallback)

Scope

• internal/usage HTTP client with injectable transport for tests
• Map API response → stable output struct used by the CLI
• Sensible timeouts (claude-hud default ~15s)
• Optional short-lived cache (nice-to-have; claude-hud uses ~5 min file cache for HUD — CLI may skip or use a simpler in-process/optional cache)
• Handle 401/403/429 and network errors with clear messages
• Respect “custom base URL ⇒ usage N/A” behavior if applicable

Acceptance criteria

• Successful call produces data matching the CLI JSON contract
• Table tests with recorded/fake HTTP responses (no network in unit tests)
• Error paths covered (unauthorized, rate limit, timeout, empty body)
• No dependency on ~/.claude/plugins/cache/**/claude-hud/**

Out of scope

• HUD rendering / statusline integration
• Mutating credentials or performing OAuth login