feat: configure extension policies and plan directory

[moisgobg]

Key Changes:

• Adopts plan.directory: Configures Conductor to store planning artifacts within the templates/ directory. This ensures that the
  agent's work remains local to the extension context rather than relying on system-wide temporary folders.
• Implements policyPaths: Points the CLI to templates/policies/ for security rules. This replaces the previous hardcoded convention
  with a configurable approach, allowing Conductor to maintain its modular project structure while still enforcing tool-use guardrails.

1. Policy Engine Foundation: Based on active PR #20049 (Add support for policy engine in extensions google-gemini/gemini-cli#20049), which established the secure framework for extension-contributed policies.
2. Plan Directory Logic: Based on active PR #20354 (feat(extensions): implement generic settings reconciliation for extensions google-gemini/gemini-cli#20354), which enabled extensions to override the global plan directory setting.