Add /branch command for conversation branching (fixes #16)

- Add /branch slash command to create conversation branches
- New UserMessageSelectorComponent shows all user messages chronologically
- Selecting a message creates new session with messages before selection
- Selected message is placed in editor for modification/resubmission
- SessionManager.createBranchedSession() creates new session files
- Updated README.md and CHANGELOG.md with /branch documentation

packages/coding-agent/src/tui/user-message-selector.ts

@@ -0,0 +1,158 @@
+import { type Component, Container, Spacer, Text } from "@mariozechner/pi-tui";
+import chalk from "chalk";
+
+/**
+ * Dynamic border component that adjusts to viewport width
+ */
+class DynamicBorder implements Component {
+	private colorFn: (text: string) => string;
+
+	constructor(colorFn: (text: string) => string = chalk.blue) {
+		this.colorFn = colorFn;
+	}
+
+	render(width: number): string[] {
+		return [this.colorFn("─".repeat(Math.max(1, width)))];
+	}
+}
+
+interface UserMessageItem {
+	index: number; // Index in the full messages array
+	text: string; // The message text
+	timestamp?: string; // Optional timestamp if available
+}
+
+/**
+ * Custom user message list component with selection
+ */
+class UserMessageList implements Component {
+	private messages: UserMessageItem[] = [];
+	private selectedIndex: number = 0;
+	public onSelect?: (messageIndex: number) => void;
+	public onCancel?: () => void;
+	private maxVisible: number = 10; // Max messages visible
+
+	constructor(messages: UserMessageItem[]) {
+		// Store messages in chronological order (oldest to newest)
+		this.messages = messages;
+		// Start with the last (most recent) message selected
+		this.selectedIndex = Math.max(0, messages.length - 1);
+	}
+
+	render(width: number): string[] {
+		const lines: string[] = [];
+
+		if (this.messages.length === 0) {
+			lines.push(chalk.gray("  No user messages found"));
+			return lines;
+		}
+
+		// Calculate visible range with scrolling
+		const startIndex = Math.max(
+			0,
+			Math.min(this.selectedIndex - Math.floor(this.maxVisible / 2), this.messages.length - this.maxVisible),
+		);
+		const endIndex = Math.min(startIndex + this.maxVisible, this.messages.length);
+
+		// Render visible messages (2 lines per message + blank line)
+		for (let i = startIndex; i < endIndex; i++) {
+			const message = this.messages[i];
+			const isSelected = i === this.selectedIndex;
+
+			// Normalize message to single line
+			const normalizedMessage = message.text.replace(/\n/g, " ").trim();
+
+			// First line: cursor + message
+			const cursor = isSelected ? chalk.blue("› ") : "  ";
+			const maxMsgWidth = width - 2; // Account for cursor
+			const truncatedMsg = normalizedMessage.substring(0, maxMsgWidth);
+			const messageLine = cursor + (isSelected ? chalk.bold(truncatedMsg) : truncatedMsg);
+
+			lines.push(messageLine);
+
+			// Second line: metadata (position in history)
+			const position = i + 1;
+			const metadata = `  Message ${position} of ${this.messages.length}`;
+			const metadataLine = chalk.dim(metadata);
+			lines.push(metadataLine);
+			lines.push(""); // Blank line between messages
+		}
+
+		// Add scroll indicator if needed
+		if (startIndex > 0 || endIndex < this.messages.length) {
+			const scrollInfo = chalk.gray(`  (${this.selectedIndex + 1}/${this.messages.length})`);
+			lines.push(scrollInfo);
+		}
+
+		return lines;
+	}
+
+	handleInput(keyData: string): void {
+		// Up arrow - go to previous (older) message
+		if (keyData === "\x1b[A") {
+			this.selectedIndex = Math.max(0, this.selectedIndex - 1);
+		}
+		// Down arrow - go to next (newer) message
+		else if (keyData === "\x1b[B") {
+			this.selectedIndex = Math.min(this.messages.length - 1, this.selectedIndex + 1);
+		}
+		// Enter - select message and branch
+		else if (keyData === "\r") {
+			const selected = this.messages[this.selectedIndex];
+			if (selected && this.onSelect) {
+				this.onSelect(selected.index);
+			}
+		}
+		// Escape - cancel
+		else if (keyData === "\x1b") {
+			if (this.onCancel) {
+				this.onCancel();
+			}
+		}
+		// Ctrl+C - cancel
+		else if (keyData === "\x03") {
+			if (this.onCancel) {
+				this.onCancel();
+			}
+		}
+	}
+}
+
+/**
+ * Component that renders a user message selector for branching
+ */
+export class UserMessageSelectorComponent extends Container {
+	private messageList: UserMessageList;
+
+	constructor(messages: UserMessageItem[], onSelect: (messageIndex: number) => void, onCancel: () => void) {
+		super();
+
+		// Add header
+		this.addChild(new Spacer(1));
+		this.addChild(new Text(chalk.bold("Branch from Message"), 1, 0));
+		this.addChild(new Text(chalk.dim("Select a message to create a new branch from that point"), 1, 0));
+		this.addChild(new Spacer(1));
+		this.addChild(new DynamicBorder());
+		this.addChild(new Spacer(1));
+
+		// Create message list
+		this.messageList = new UserMessageList(messages);
+		this.messageList.onSelect = onSelect;
+		this.messageList.onCancel = onCancel;
+
+		this.addChild(this.messageList);
+
+		// Add bottom border
+		this.addChild(new Spacer(1));
+		this.addChild(new DynamicBorder());
+
+		// Auto-cancel if no messages or only one message
+		if (messages.length <= 1) {
+			setTimeout(() => onCancel(), 100);
+		}
+	}
+
+	getMessageList(): UserMessageList {
+		return this.messageList;
+	}
+}