feat(coding-agent): implement hooks system

- Add hooks infrastructure in core/hooks/ (loader, runner, types)
- HookUIContext interface with mode-specific implementations
- Interactive mode: TUI-based selector/input/confirm dialogs
- RPC mode: JSON protocol for hook UI requests/responses
- Print mode: no-op UI context (hooks run but can't prompt)
- AgentSession.branch() now async, returns { selectedText, skipped }
- Settings: hooks[] and hookTimeout configuration
- Export hook types from package for hook authors

Based on PR #147 proposal, adapted for new architecture.

packages/coding-agent/src/modes/interactive/components/hook-input.ts

@@ -0,0 +1,64 @@
+/**
+ * Simple text input component for hooks.
+ */
+
+import { Container, 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 {
+		// Enter
+		if (keyData === "\r" || keyData === "\n") {
+			this.onSubmitCallback(this.input.getValue());
+			return;
+		}
+
+		// Escape to cancel
+		if (keyData === "\x1b") {
+			this.onCancelCallback();
+			return;
+		}
+
+		// Forward to input
+		this.input.handleInput(keyData);
+	}
+}