Add policy hot reload support

[lleonardo-franco]

Summary

• Add PolicyWatcher support for file, URL, and custom provider policy sources.
• Add TealEngine runtime reload APIs with validated atomic swaps, reload events, watcher lifecycle helpers, and versioned evaluation metadata.
• Cover manual reloads, invalid policy rejection, delayed provider swaps, watched file hot reloads during active evaluations, and HTTP polling validators.

Validation

• git diff --check
• npx tsc --noEmit
• npm run build:types
• npm test -- --runTestsByPath src/core/engine/tests/PolicyWatcher.test.ts --runInBand --no-watchman
• npm test -- --runInBand --no-watchman (94 suites, 2365 tests passed; Jest printed its existing open-handle warning after success)

Resolves agentguard-ai/tealtiger#121