Add type guards for tool_result event narrowing

- Export isBashToolResult, isReadToolResult, etc. type guards
- Update hooks.md with type guard usage examples
- Document custom tool handling in hooks.md

packages/coding-agent/CHANGELOG.md

@@ -11,6 +11,7 @@
   - `ToolResultEventResult.result` renamed to `ToolResultEventResult.text` (removed), use `content` instead
   - Hook handlers returning `{ result: "..." }` must change to `{ content: [{ type: "text", text: "..." }] }`
   - Built-in tool details types exported: `BashToolDetails`, `ReadToolDetails`, `GrepToolDetails`, `FindToolDetails`, `LsToolDetails`, `TruncationResult`
+  - Type guards exported for narrowing: `isBashToolResult`, `isReadToolResult`, `isEditToolResult`, `isWriteToolResult`, `isGrepToolResult`, `isFindToolResult`, `isLsToolResult`
 
 ### Changed