feat(hooks): config_change event — fires on /reload

[emal-avala]

Summary

Adds a ConfigChange hook event that fires when on-disk extensions are reloaded via /reload. External dashboards, CI watchers, and alerting pipelines can now detect "a new skill file just landed" or "MCP server count dropped" without polling the filesystem.

Context payload

{
  "skill_count": 24,
  "agent_count": 3,
  "hook_count": 2,
  "mcp_count": 1
}

Wiring

• HookEvent::ConfigChange variant with snake_case serde
• fire_config_change_hooks(skill, agent, hook, mcp) async method on QueryEngine
• /reload handler fires after the extension rescan + prompt-cache invalidation completes, using the existing block_on bridge (matches /compact, /cd, /add-dir precedents)
• HOOK_EVENT_CATALOG, format_hook_event, parse_hook_event updated
• hooks/mod.rs module docs updated

Test plan

• cargo clippy --workspace --tests --no-deps -- -D warnings — clean
• cargo test -p agent-code-lib --lib hooks::tests — 10 pass (9 prior + 1 new)
• cargo test -p agent-code-lib --lib config::schema::tests::hook_event_serde_roundtrip_config_change — pass
• cargo test -p agent-code --bin agent commands::tests::parse_hook_event — 11 pass (10 prior + 1 new)

3 new tests:

1. Schema serde round-trip (config_change ↔ "config_change")
2. run_hooks dispatches for ConfigChange
3. parse_hook_event accepts "config_change" / "config-change"

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