Improve commit-pathspec benchmark gating and fast-path coverage

[svarlamov]

Summary

• add AI pathspec hotspot benchmark gate to perf workflows (smoke + nightly)
• benchmark plain git vs git-ai using ratio threshold (<=2x), with multi-run median stability
• harden benchmark plain-git baseline resolution to avoid wrapper contamination
• optimize post-commit pathspec handling for attribution-empty AI entries and skip unnecessary stats work
• add regression tests for empty-entry filtering, untracked pathspec handling, and post-commit fast-path prompt preservation
• normalize snapshot tests across git_ai_version variants

Validation

• cargo fmt -- --check
• cargo clippy
• cargo test test_post_commit_fast_path_preserves_prompt_metrics_for_empty_ai_entries -- --nocapture
• cargo test --test commit_post_stats_benchmark benchmark_commit_post_command_ai_pathspec_hotspot -- --ignored --nocapture

---

[git-ai-cloud]

Stats powered by Git AI

🧠 you    ████████████████████  100%
🤖 ai     ░░░░░░░░░░░░░░░░░░░░  0%

More stats

• 0.0 lines generated for every 1 accepted
• 0 seconds waiting for AI

AI code tracked with git-ai

[git-ai-cloud-dev]

Stats powered by Git AI

🧠 you    ████████████████████  100%
🤖 ai     ░░░░░░░░░░░░░░░░░░░░  0%

More stats

• 0.0 lines generated for every 1 accepted
• 0 seconds waiting for AI

AI code tracked with git-ai

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 7 additional findings.