perf(snapshot): add mutex lock, incremental add, and batched revert
Port snapshot optimizations from upstream OpenCode to reduce git process
spawning and prevent concurrent corruption:
- Per-gitdir mutex lock (upstream #17878, v1.3.0): serializes concurrent
snapshot operations to prevent race conditions
- Incremental git add (upstream #17878, v1.3.0): replaces naive `git add .`
with diff-files + ls-files + size filtering, skipping files >2MB
- Batched revert (upstream #20564, v1.3.14): groups up to 100 files per
git checkout call with path-conflict detection, falling back to
single-file revert on failure
Benchmarks (200 files):
- revert: 2,851ms → 191ms (15x faster, 8x fewer git processes)
- concurrent track: 2/5 succeed → 5/5 succeed (corruption fixed)
Johnny Amancio
afe950dc0c