Add watchdog-based selective hot-reload with safe patch application

[farukk77]

What

This PR upgrades runtime hot-reload from polling-based mtime checks to watchdog file-system events (with polling fallback), and moves “partial vs full reload” decisions into the runtime via a field-level patch.

Why

• Faster and more reliable config reload triggers (no polling delay when watchdog is available)
• Clear separation of concerns: watcher only signals “changed”, runtime decides what to do
• Safer reload semantics: partial updates only for whitelisted prompt/governance fields; full reload otherwise
• More robust runtime config writes on Windows via atomic replace

Changes

• Add watchdog dependency and an async-friendly debounced file watcher ( AsyncFileWatcher )
• Single-mode: patch-based reload decision + watchdog integration + polling fallback
• Multi-mode: same patch-based pattern + watchdog integration + polling fallback
• ConfigProvider: write runtime config atomically using os.replace and json5.dump
• Tests updated/added to cover watchdog-triggered reload and reload safety

Key files

• src/runtime/hot_reload/watcher.py
• src/runtime/single_mode/cortex.py
• src/runtime/multi_mode/cortex.py
• • src/providers/config_provider.py
• tests/runtime/**/test_cortex*.py

Verification

• Ran targeted hot-reload related tests:
  • tests/runtime/single_mode/test_cortex.py
  • tests/runtime/multi_mode/test_cortex.py
  • tests/runtime/multi_mode/test_cortex_mode_transition.py