docs: add global keyboard shortcuts reference (#543)

[Gautam5471]

Summary

Implements #543 by adding docs/KEYBOARD_SHORTCUTS.md, an evidence-based reference for PACT OS keyboard shortcuts, platform differences, modal navigation, and accessibility behavior.

Coverage

• Documents verified Cmd/Ctrl + K command palette behavior.
• Explains keyboard shortcuts guide activation and input-field exceptions.
• Documents command palette navigation using arrow keys, Home, End, and Enter.
• Covers modal dismissal, focus trapping, and focus restoration where implemented.
• Documents component-specific keyboard interactions and accessibility patterns.
• Distinguishes implemented shortcuts from planned or reserved shortcuts, including the Focus Timer Space shortcut limitation.
• Includes platform notes for macOS and Windows/Linux.

Validation

• git diff --cached --check — passed.
• Markdown link audit — passed.
• npm run lint — passed.
• npx tsc --noEmit — passed.
• npm test — 75/75 suites passed.

Documentation-only change; no application code or dependencies modified.

Closes #543

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