feat(source): add Amp adapter

[ayagmar]

Objective

Add an amp source adapter for local Amp thread files with support for both aggregated ledger usage and per-message fallback parsing.

Source shape to support

• directory-backed JSON source under ~/.local/share/amp/threads/
• thread files named T-*.json
• preferred usage source is an aggregated ledger/event list when present
• fallback usage source is assistant message usage embedded in the same thread file
• explicit credits/cost may already be present in the source data

Implementation plan

1. Adapter file

   • add src/sources/amp/amp-source-adapter.ts
   • implement SourceAdapter with id = 'amp'

2. Discovery

   • use discoverFiles(..., { extension: '.json' }) plus a T-*.json filename filter or equivalent deterministic discovery helper
   • keep the adapter directory-backed and reachable with --source-dir amp=...

3. Parsing

   • parse the thread JSON once per file
   • prefer aggregated ledger/events when available because they represent the cleanest usage signal
   • if no usable ledger events exist, fall back to assistant message usage records from the same file
   • preserve explicit cost when present
   • infer provider from model names conservatively
   • use thread-created time and then file mtime as timestamp fallbacks when event timestamps are missing
   • skip zero-token records so empty bookkeeping rows do not create usage events
   • use parseFileWithDiagnostics() if malformed thread files or invalid usage sections should surface skipped-row reasons

4. Wiring

   • register amp in src/sources/create-default-adapters.ts
   • rely on generic --source-dir support rather than adding a dedicated path flag
   • update supported-source registry/help expectations

5. Tests and docs

   • add tests/sources/amp-source-adapter.test.ts
   • cover both ledger-first and message-fallback behavior
   • update shared registry/help tests and README.md

Code paths to touch

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

Verification

• parser fixtures cover:
  • ledger-backed usage events
  • assistant-message fallback when no usable ledger exists
  • timestamp fallback order
  • zero-token rows skipped
  • explicit cost preserved
• llm-usage daily --source amp --source-dir amp=<fixture-dir> works
• supported-source help output and registry tests are updated
• full validation passes:
  • pnpm run lint
  • pnpm run typecheck
  • pnpm run test
  • pnpm run format:check