[Example] Add policy hot-reload demo

[Acacian]

What

Add an example script (examples/hot_reload_demo.py) demonstrating policy hot-reload and policy merge.

Why

runtime.update_policy() and Policy.from_yaml_files() were added in v0.1.3 but lack a standalone example. These features are important for production deployments where policies change without restarts.

Expected behavior

1. Start with a permissive policy (auto-approve reads)
2. Execute some actions
3. Hot-reload a stricter policy (block deletes)
4. Show the same actions now get blocked
5. Demonstrate Policy.from_yaml_files() for merging base + override policies

Acceptance criteria

• examples/hot_reload_demo.py runs without errors
• Demonstrates runtime.update_policy()
• Demonstrates Policy.from_yaml_files() or policy.merge()
• Added to examples/README.md

Difficulty

Beginner — straightforward API usage.