feat(source): add Mux adapter

[ayagmar]

Objective

Add a mux source adapter for workspace session-usage summaries.

Source shape to support

• directory-backed JSON source under ~/.mux/sessions/**/session-usage.json
• one file can contain usage for multiple models in byModel
• token buckets include input, cache read, cache write, output, and reasoning
• explicit cost may exist per bucket rather than as one top-level total

Implementation plan

1. Adapter file

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

2. Discovery

   • discover session-usage.json files deterministically
   • keep the source directory-backed and reachable via --source-dir mux=...

3. Parsing

   • parse the file once and emit one UsageEvent per byModel entry
   • map all available token buckets through createUsageEvent, preserving reasoning/cache buckets intact
   • if the model key is encoded as provider:model, split provider and model before normalization
   • sum explicit per-bucket costs into costUsd for the emitted event and mark costMode: 'explicit'
   • fall back to file mtime if lastRequest.timestamp is missing
   • derive sessionId from the workspace/session directory name
   • skip zero-token model entries so empty bookkeeping does not create rows

4. Wiring

   • register mux in src/sources/create-default-adapters.ts
   • keep this on generic --source-dir support
   • update supported-source count/order/help expectations

5. Tests and docs

   • add tests/sources/mux-source-adapter.test.ts
   • update shared registry/help tests and README.md

Code paths to touch

• src/sources/mux/mux-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/mux-source-adapter.test.ts
• tests/sources/create-default-adapters.test.ts
• tests/cli/create-cli.test.ts
• README.md

Verification

• parser fixtures cover:
  • multi-model byModel files
  • provider/model split from combined keys
  • explicit cost summed from per-bucket cost fields
  • missing lastRequest.timestamp fallback
  • zero-token entries skipped
• llm-usage daily --source mux --source-dir mux=<fixture-dir> works
• full validation passes:
  • pnpm run lint
  • pnpm run typecheck
  • pnpm run test
  • pnpm run format:check