docs: show timeout in frontmatter-requirements.md hooks example

[philoserf]

Problem

The hooks example in frontmatter-requirements.md shows basic hook structure but omits the timeout option:

hooks:
  - event: "AgentStart"
    command: "./scripts/prepare-agent.sh"
    description: "Prepare the agent environment"

Users may not realize timeout is configurable and important for long-running hooks.

Solution

Expand the example to include timeout:

hooks:
  - event: "AgentStart"
    command: "./scripts/prepare-agent.sh"
    description: "Prepare the agent environment"
    timeout: 10000  # 10 seconds (default: 5000ms)

Add a note explaining timeout behavior:

**Timeout:** Hooks have a default timeout of 5 seconds. For longer operations:
- Set explicit `timeout` in milliseconds
- Consider async patterns for very long operations
- Hooks that timeout are treated as failures (may block the operation)

Files to Update

• references/frontmatter-requirements.md

Impact

• Priority: Low
• Effort: Low
• User Impact: Prevents unexpected hook failures from timeouts

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Review context: commit f2fe29b (agent hooks documentation)

[philoserf]

Closing: the timeout field is already shown in the hooks example in references/frontmatter-requirements.md (line 136).