Add circuit breaker for hydration status

[tyrielv]

Auto-disable hydration status computation after 3 failures per UTC day. Resets on new day or GVFS version change. Checked in hooks (pre-command) and GitStatusCache (background) but NOT in manual gvfs health.

• New HydrationStatusCircuitBreaker class using System.IO directly for file-link compatibility with GVFS.Hooks
• Marker file at .gvfs/gitStatusCache/HydrationStatusDisabled.dat
• 12 unit tests covering trip, reset, parse, and edge cases

[KeithIsSleeping]

see below

[KeithIsSleeping]

(Corrected re-post — the original review had rendering issues with backtick escaping)

Well-designed safety valve. The circuit breaker pattern is a good fit here, and the implementation is solid. A couple of notes:

1. Minor TOCTOU near midnight — IsDisabled() reads the date with DateTime.UtcNow and RecordFailure() reads it again independently. If called right at midnight, IsDisabled could see "2026-03-17" and RecordFailure could see "2026-03-18", effectively writing a reset. Extremely unlikely to matter in practice, but passing the timestamp as a parameter would make it both consistent and more testable.

2. Hooks path uses NullTracer.Instance — Circuit breaker warnings in the pre-command hook path are silently discarded. Is that intentional? The hooks are lightweight so I understand the tradeoff, but it means you can't tell from logs that the circuit breaker fired during a git status hook invocation.

3. Design choice feedback — Good call avoiding PhysicalFileSystem dependency for hooks file-link compatibility. The exclusive file locking with graceful skip-if-locked fallback is clean. Version-based reset is clever — a GVFS update that fixes the underlying bug automatically re-enables the feature.

Test coverage is thorough (12 tests covering trip, reset, parse edge cases, version change, day rollover). Looks good.