docs: add migration guide for adding hooks to existing agents

[philoserf]

Problem

Users with existing agents may want to add hooks but the documentation focuses on creating new agents with hooks. There's no guidance on retrofitting hooks to existing agents.

Solution

Add a "Adding Hooks to Existing Agents" section to the agent-authoring SKILL.md:

## Adding Hooks to Existing Agents

### Migration Checklist

1. **Identify hook opportunities** - Review your agent's typical workflow:
   - What operations could benefit from validation?
   - Where would logging/auditing be valuable?
   - Are there operations that should trigger notifications?

2. **Start with non-blocking hooks** - Begin with logging/observability:
   ```yaml
   hooks:
     - event: "AgentStart"
       command: "echo 'Agent started' >> /tmp/agent.log"
       description: "Log agent activity (non-blocking)"

3. Add blocking hooks incrementally - Test one at a time:

   hooks:
     - event: "Bash"
       matcher:
         command: "rm -rf *"
       command: "./scripts/confirm-destructive.sh"
       description: "Require confirmation for destructive commands"

4. Test in isolation - Verify hooks work before deploying:

   # Test hook script directly
   ./scripts/my-hook.sh
   echo $?  # Should be 0 for pass, non-zero for block

Common Migration Patterns

Current State	Hook to Add	Benefit
No logging	AgentStart/Stop hooks	Audit trail
Manual validation	Matcher-based hooks	Automated checks
Post-hoc review	Tool-specific hooks	Real-time validation

Backward Compatibility

Hooks are optional - existing agents work unchanged. Add hooks gradually without disrupting current workflows.

## Files to Update

- `skills/agent-authoring/SKILL.md`

## Impact

- **Priority:** Low
- **Effort:** Medium
- **User Impact:** Enables adoption of hooks for existing agent configurations

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*Review context: commit f2fe29b (agent hooks documentation)*