Merge hooks and custom-tools into unified extensions system (#454)

Breaking changes:
- Settings: 'hooks' and 'customTools' arrays replaced with 'extensions'
- CLI: '--hook' and '--tool' flags replaced with '--extension' / '-e'
- API: HookMessage renamed to CustomMessage, role 'hookMessage' to 'custom'
- API: FileSlashCommand renamed to PromptTemplate
- API: discoverSlashCommands() renamed to discoverPromptTemplates()
- Directories: commands/ renamed to prompts/ for prompt templates

Migration:
- Session version bumped to 3 (auto-migrates v2 sessions)
- Old 'hookMessage' role entries converted to 'custom'

Structural changes:
- src/core/hooks/ and src/core/custom-tools/ merged into src/core/extensions/
- src/core/slash-commands.ts renamed to src/core/prompt-templates.ts
- examples/hooks/ and examples/custom-tools/ merged into examples/extensions/
- docs/hooks.md and docs/custom-tools.md merged into docs/extensions.md

New test coverage:
- test/extensions-runner.test.ts (10 tests)
- test/extensions-discovery.test.ts (26 tests)
- test/prompt-templates.test.ts

packages/coding-agent/src/modes/interactive/components/custom-editor.ts

@@ -12,8 +12,8 @@ export class CustomEditor extends Editor {
 	public onEscape?: () => void;
 	public onCtrlD?: () => void;
 	public onPasteImage?: () => void;
-	/** Handler for hook-registered shortcuts. Returns true if handled. */
-	public onHookShortcut?: (data: string) => boolean;
+	/** Handler for extension-registered shortcuts. Returns true if handled. */
+	public onExtensionShortcut?: (data: string) => boolean;
 
 	constructor(theme: EditorTheme, keybindings: KeybindingsManager) {
 		super(theme);
@@ -28,8 +28,8 @@ export class CustomEditor extends Editor {
 	}
 
 	handleInput(data: string): void {
-		// Check hook-registered shortcuts first
-		if (this.onHookShortcut?.(data)) {
+		// Check extension-registered shortcuts first
+		if (this.onExtensionShortcut?.(data)) {
 			return;
 		}