WIP Add manual watch mode for restart control

[xiantang]

close #804
Add watch_mode configuration option ('auto' or 'manual') to allow users to control when Air restarts their application. In manual mode, the app only restarts when the user presses 'r', ignoring file changes. This is useful for apps with slow startup times (e.g., database connections).

Changes:

• Add watch_mode field to build config (default: 'auto')
• Add keyboard listener in main.go to detect 'r' key press
• Add manualRestartCh channel to Engine for triggering restarts
• Ignore file change events when in manual mode
• Add tests for watch_mode configuration
• Update air_example.toml with watch_mode documentation

[codecov]

Codecov Report

❌ Patch coverage is 39.47368% with 23 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
main.go	0.00%	14 Missing ⚠️
runner/engine.go	60.86%	8 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
runner/config.go	74.90% <100.00%> (+0.09%)	⬆️
runner/engine.go	59.39% <60.86%> (-1.10%)	⬇️
main.go	0.00% <0.00%> (ø)

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