feat(ai): report Cursor account usage

[riverpilot]

Summary

• Add Cursor OAuth and API-key usage reporting through the shared usage APIs.
• Render Cursor request and billing quotas in usage views and cover parsing/provider behavior with tests.

Test plan

• bun test packages/ai/test/cursor-usage.test.ts
• bunx biome check on changed TypeScript files
• bun test coding-agent usage suites (blocked locally: missing packages/natives pi_natives.darwin-arm64.node)
• coding-agent type-check (blocked locally: missing header-generator dependency)

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[roboomp]

P2 — this adds Cursor account-usage reporting, so the new provider-facing behavior needs an explicit maintainer call rather than a default merge.
The staged inline findings identify the concrete correctness and convention issues that should be resolved before landing.
Thanks for the contribution, @riverpilot.