Persist inference errors on empty partials and honest exec turn counts

[TheGreatAxios]

Summary

• partial.jsonl records for inference-error now include error (category, message, statusCode when present), including cycles that streamed no text.
• Exec mid-run and terminal run.json snapshots read turnsUsed from runSink.getTurnCount() (and persist on each turn boundary) instead of the initial zero.
• Docs note that latest is a symlink, not a session, so naive globs of the project sessions directory must skip it.

Verification

• bun run typecheck passes
• bun test src/session/stream-journal.test.ts src/session/run-sink.test.ts — 30 pass
• Pre-commit typecheck and build pass

Fixes CL-6917

[linear-code]

CL-6917