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/hook-input.ts

@@ -1,65 +0,0 @@
-/**
- * Simple text input component for hooks.
- */
-
-import { Container, getEditorKeybindings, Input, Spacer, Text } from "@mariozechner/pi-tui";
-import { theme } from "../theme/theme.js";
-import { DynamicBorder } from "./dynamic-border.js";
-
-export class HookInputComponent extends Container {
-	private input: Input;
-	private onSubmitCallback: (value: string) => void;
-	private onCancelCallback: () => void;
-
-	constructor(
-		title: string,
-		_placeholder: string | undefined,
-		onSubmit: (value: string) => void,
-		onCancel: () => void,
-	) {
-		super();
-
-		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 input
-		this.input = new Input();
-		this.addChild(this.input);
-
-		this.addChild(new Spacer(1));
-
-		// Add hint
-		this.addChild(new Text(theme.fg("dim", "enter submit  esc cancel"), 1, 0));
-
-		this.addChild(new Spacer(1));
-
-		// Add bottom border
-		this.addChild(new DynamicBorder());
-	}
-
-	handleInput(keyData: string): void {
-		const kb = getEditorKeybindings();
-		// Enter
-		if (kb.matches(keyData, "selectConfirm") || keyData === "\n") {
-			this.onSubmitCallback(this.input.getValue());
-			return;
-		}
-
-		// Escape or Ctrl+C to cancel
-		if (kb.matches(keyData, "selectCancel")) {
-			this.onCancelCallback();
-			return;
-		}
-
-		// Forward to input
-		this.input.handleInput(keyData);
-	}
-}