[Example] Add RuntimeHooks observability example

[Acacian]

What

Add an example script (examples/hooks_demo.py) showing how to use RuntimeHooks for observability.

Why

RuntimeHooks (on_decision, on_approval, on_execute) were added in v0.1.3 but don't have a standalone example. This is one of Aegis's most useful features for production monitoring.

Expected example

from aegis import Action, Policy, Runtime
from aegis.runtime.engine import RuntimeHooks

async def on_decision(decision):
    print(f"[DECISION] {decision.action.type} → {decision.approval.value}")

async def on_execute(result):
    print(f"[EXECUTED] {result.action.type} → {result.status.value}")

hooks = RuntimeHooks(on_decision=on_decision, on_execute=on_execute)
runtime = Runtime(executor=..., policy=..., hooks=hooks)

Acceptance criteria

• examples/hooks_demo.py runs without errors
• Shows all 3 hook types (on_decision, on_approval, on_execute)
• Added to examples/README.md
• Uses AutoApprovalHandler so it runs non-interactively

Difficulty

Beginner — copy patterns from existing examples.