v0.7.1: Enable Claude extra usage spend tracking (alpha feature)

[danielithomas]

Summary

Claude's extra usage spend data (dollar amounts for usage beyond the subscription cap) is not available via any stable API. The data is visible at claude.ai/settings/usage in the browser but has no documented REST endpoint.

This has been an open question since project inception (OQ-001) and was previously deferred (D-005). With the introduction of enable_alpha_features (D-053 in v0.7.0), we now have a clean mechanism to ship unstable data sources without misrepresenting their reliability.

Scope

• Implement Claude extra usage spend tracking behind enable_alpha_features = true
• Investigate available data sources:
  • Undocumented /api/oauth/usage endpoint (OQ-002 — rate-limited, may change)
  • Scraping claude.ai/settings/usage via authenticated request
  • Any new Anthropic endpoints that may have appeared
• Map extra usage data to a new UsageWindow (e.g. "Extra Usage Spend", unit: "usd")
• Alpha-sourced windows flagged with alpha: true in extras dict
• Emit stderr warning on first use: "Claude extra usage tracking is an alpha feature — it relies on undocumented interfaces that may break without notice"
• Graceful failure — if the data source is unavailable, silently skip (no error in provider output)

Relates to

• OQ-001 — How to surface Claude extra usage spend data?
• D-005 — Claude extra usage spend available as alpha feature
• D-053 — enable_alpha_features flag for unstable data sources
• OQ-002 — Is the /api/oauth/usage endpoint stable enough to depend on?
• OQ-008 — What does utilization > 100 look like in Claude's API?

Acceptance criteria

• Extra usage spend appears as a UsageWindow when enable_alpha_features = true
• No effect when enable_alpha_features = false (default)
• Stderr warning on first use per session
• Graceful degradation if data source is unavailable
• Tests with mocked responses (both success and failure paths)
• README alpha features section updated with Claude extra usage details

Milestone

v0.7.1 (patch release after v0.7.0 Ollama provider)