feat: stream-json NDJSON parser for real-time E2E progress

Switch session-runner from buffered `--output-format json` to streaming
`--output-format stream-json --verbose`. Parses NDJSON line-by-line for
real-time tool-by-tool progress on stderr during 3-5 min E2E runs.

- Extract testable `parseNDJSON()` function (pure, no I/O)
- Count turns per assistant event (not per text block)
- Add `transcript: any[]` to SkillTestResult, remove dead `messages` field
- Reconstruct allText from transcript for browse error scanning
- 8 unit tests for parser (malformed lines, empty input, turn counting)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>