perf(sessions): sub-second /chat loading for large session histories

[N1kky-wed]

Summary

The /chat command took 25+ seconds to load with large session histories.
This PR reduces load time to 634ms on a real-world benchmark of 59 sessions / 2.3GB of JSONL.

Details

Three compounding bottlenecks eliminated:

1. chatRecordingService.ts: New ultra-fast preview path uses raw fd.read()
   calls (64KB head + 128KB tail). Core metadata is extracted via regex directly
   on buffers, while the first user message is extracted via line-based JSON parsing
   to robustly handle nested structures — without streaming the full file.

2. sessionUtils.ts: Bulk wc -l across all files in a single subprocess call
   (batched at 100 files) replaces per-file subprocess calls. Windows fallback uses
   batch-limited (20 concurrent) Node.js buffer scanning to avoid FD exhaustion.

3. utils.ts: Stable secondary sort by startTime when lastUpdated ties, fixing
   inconsistent session ordering. Filename timestamp used as reliable startTime
   fallback instead of mtime.

Note: The 25.62s benchmark was recorded on a fresh environment. In practice,
load times frequently exceed 60 seconds on warmed systems with large histories.

Related Issues

Fixes #27027

How to Validate

1. Generate a large session history (30+ sessions with substantial conversation history recommended — the more messages per session, the larger the files and the more pronounced the improvement)
2. Run /chat and compare load time against main branch
3. Verify session list ordering is consistent across multiple opens
4. On Linux: load time should be under 1 second for typical histories

Note: Windows fallback path (batch-limited buffer scanning) is implemented
but not benchmarked on hardware — community validation welcome.

Pre-Merge Checklist

• Updated relevant documentation and README (if needed)
• Added/updated tests (if needed)
• Noted breaking changes (if any)
• Validated on required platforms/methods:
  • MacOS
    • npm run
    • npx
    • Docker
    • Podman
    • Seatbelt
  • Windows
    • npm run
    • npx
    • Docker
  • Linux
    • npm run
    • npx
    • Docker