v0.5.3: per-key timestamps, murk scan, exec --only/--clean-env, quick-start guide

[iicky]

• Add created/updated timestamps to schema entries (set on add, import, describe)
• Add murk scan command: scan files for leaked secret values
• Add --only and --clean-env flags to murk exec for lower-exposure execution
• Add quick-start guide for teammates (docs/quick-start.md)
• Add walkdir dependency for scan file traversal
• SchemaEntry now derives Default for cleaner construction

[codecov]

Codecov Report

❌ Patch coverage is 17.14286% with 58 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.45%. Comparing base (70fa75a) to head (5b3330a).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/main.rs	0.00%	58 Missing ⚠️

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