feat(source): add Cursor cache adapter

[ayagmar]

Objective

Add a cursor source adapter for local usage CSV caches so Cursor usage can be parsed without introducing auth or sync behavior into this repository.

Source shape to support

• directory-backed cache source containing usage.csv and usage.<account>.csv
• at least two known CSV header layouts must be supported
• rows carry explicit cost
• timestamps may be full ISO datetimes or date-only values
• model strings may need conservative provider inference

Implementation plan

1. Adapter file

   • add src/sources/cursor/cursor-source-adapter.ts
   • implement SourceAdapter with id = 'cursor'
   • keep this source parse-only; do not add login, API sync, or cache-population behavior here

2. Discovery

   • treat this as a directory-backed source reachable through generic --source-dir cursor=...
   • discover the cache files deterministically and ignore unrelated files
   • account-scoped files should remain distinct inputs rather than being merged before parsing

3. Parsing

   • support both known CSV header layouts instead of assuming one fixed column order
   • preserve explicit cost through createUsageEvent({ costUsd, costMode: 'explicit' })
   • infer provider conservatively from the model string; do not overclaim fixed-provider capabilities for this source
   • keep cache write tokens derived from the delta between “input with cache write” and “input without cache write” when the file shape exposes both
   • for date-only rows, assign a stable in-day timestamp (rather than a boundary timestamp that can drift across local-day bucketing)
   • skip blank / malformed rows without aborting the whole file
   • if diagnostics are worthwhile, implement parseFileWithDiagnostics() so malformed-row counts are visible

4. Wiring

   • register cursor in src/sources/create-default-adapters.ts
   • make sure supported-source IDs and help output reflect the new source
   • do not add a bespoke --cursor-dir flag unless implementation proves generic --source-dir is insufficient

5. Tests and docs

   • add tests/sources/cursor-source-adapter.test.ts
   • update shared registry/help tests that assert supported source IDs/counts
   • document the supported cache file shapes and --source-dir cursor=... usage in README.md

Code paths to touch

• src/sources/cursor/cursor-source-adapter.ts
• src/sources/create-default-adapters.ts
• src/sources/parsing-utils.ts
• src/domain/usage-event.ts
• tests/sources/cursor-source-adapter.test.ts
• tests/sources/create-default-adapters.test.ts
• tests/cli/create-cli.test.ts
• README.md

Verification

• parser fixtures cover:
  • both CSV header layouts
  • usage.csv and usage.<account>.csv
  • blank/malformed rows skipped cleanly
  • date-only timestamp handling is stable for report bucketing
  • explicit cost is preserved
  • provider inference stays conservative
• llm-usage daily --source cursor --source-dir cursor=<fixture-dir> works
• no auth/sync commands, credentials, or network code are introduced
• full validation passes:
  • pnpm run lint
  • pnpm run typecheck
  • pnpm run test
  • pnpm run format:check