fix: enforce minimum debounce of 10ms to prevent CPU exhaustion

[echobt]

Description

This PR addresses an issue where setting watch_debounce_ms to 0 caused 100% CPU usage due to a tight polling loop. A minimum debounce value of 10ms is now enforced to prevent this denial of service condition.

Fix

• Added validation in set_watch_debounce in src/config.rs to reject values less than 10ms.
• Setting 0 now returns a clear error message instead of causing high CPU usage.

Testing

• Validated that vgrep config set watch-debounce 0 now fails with "Watch debounce must be at least 10ms".
• Ran existing tests to ensure no regressions.