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/extension-editor.ts

@@ -0,0 +1,119 @@
+/**
+ * Multi-line editor component for extensions.
+ * Supports Ctrl+G for external editor.
+ */
+
+import { spawnSync } from "node:child_process";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { Container, Editor, getEditorKeybindings, matchesKey, Spacer, Text, type TUI } from "@mariozechner/pi-tui";
+import { getEditorTheme, theme } from "../theme/theme.js";
+import { DynamicBorder } from "./dynamic-border.js";
+
+export class ExtensionEditorComponent extends Container {
+	private editor: Editor;
+	private onSubmitCallback: (value: string) => void;
+	private onCancelCallback: () => void;
+	private tui: TUI;
+
+	constructor(
+		tui: TUI,
+		title: string,
+		prefill: string | undefined,
+		onSubmit: (value: string) => void,
+		onCancel: () => void,
+	) {
+		super();
+
+		this.tui = tui;
+		this.onSubmitCallback = onSubmit;
+		this.onCancelCallback = onCancel;
+
+		// Add top border
+		this.addChild(new DynamicBorder());
+		this.addChild(new Spacer(1));
+
+		// Add title
+		this.addChild(new Text(theme.fg("accent", title), 1, 0));
+		this.addChild(new Spacer(1));
+
+		// Create editor
+		this.editor = new Editor(getEditorTheme());
+		if (prefill) {
+			this.editor.setText(prefill);
+		}
+		this.addChild(this.editor);
+
+		this.addChild(new Spacer(1));
+
+		// Add hint
+		const hasExternalEditor = !!(process.env.VISUAL || process.env.EDITOR);
+		const hint = hasExternalEditor
+			? "ctrl+enter submit  esc cancel  ctrl+g external editor"
+			: "ctrl+enter submit  esc cancel";
+		this.addChild(new Text(theme.fg("dim", hint), 1, 0));
+
+		this.addChild(new Spacer(1));
+
+		// Add bottom border
+		this.addChild(new DynamicBorder());
+	}
+
+	handleInput(keyData: string): void {
+		// Ctrl+Enter to submit
+		if (keyData === "\x1b[13;5u" || keyData === "\x1b[27;5;13~") {
+			this.onSubmitCallback(this.editor.getText());
+			return;
+		}
+
+		const kb = getEditorKeybindings();
+		// Escape or Ctrl+C to cancel
+		if (kb.matches(keyData, "selectCancel")) {
+			this.onCancelCallback();
+			return;
+		}
+
+		// Ctrl+G for external editor (keep matchesKey for this app-specific action)
+		if (matchesKey(keyData, "ctrl+g")) {
+			this.openExternalEditor();
+			return;
+		}
+
+		// Forward to editor
+		this.editor.handleInput(keyData);
+	}
+
+	private openExternalEditor(): void {
+		const editorCmd = process.env.VISUAL || process.env.EDITOR;
+		if (!editorCmd) {
+			return;
+		}
+
+		const currentText = this.editor.getText();
+		const tmpFile = path.join(os.tmpdir(), `pi-extension-editor-${Date.now()}.md`);
+
+		try {
+			fs.writeFileSync(tmpFile, currentText, "utf-8");
+			this.tui.stop();
+
+			const [editor, ...editorArgs] = editorCmd.split(" ");
+			const result = spawnSync(editor, [...editorArgs, tmpFile], {
+				stdio: "inherit",
+			});
+
+			if (result.status === 0) {
+				const newContent = fs.readFileSync(tmpFile, "utf-8").replace(/\n$/, "");
+				this.editor.setText(newContent);
+			}
+		} finally {
+			try {
+				fs.unlinkSync(tmpFile);
+			} catch {
+				// Ignore cleanup errors
+			}
+			this.tui.start();
+			this.tui.requestRender();
+		}
+	}
+}