Add bash mode for executing shell commands

- Add ! prefix in TUI editor to execute shell commands directly
- Output streams in real-time and is added to LLM context
- Supports multiline commands, cancellation (Escape), truncation
- Preview mode shows last 20 lines, Ctrl+O expands full output
- Commands persist in session history as bashExecution messages
- Add bash command to RPC mode via {type:'bash',command:'...'}
- Add RPC tests for bash command execution and context inclusion
- Update docs: rpc.md, session.md, README.md, CHANGELOG.md

Closes #112

Co-authored-by: Markus Ylisiurunen <markus.ylisiurunen@gmail.com>

packages/coding-agent/src/messages.ts

@@ -0,0 +1,102 @@
+/**
+ * Custom message types and transformers for the coding agent.
+ *
+ * Extends the base AppMessage type with coding-agent specific message types,
+ * and provides a transformer to convert them to LLM-compatible messages.
+ */
+
+import type { AppMessage } from "@mariozechner/pi-agent-core";
+import type { Message } from "@mariozechner/pi-ai";
+
+// ============================================================================
+// Custom Message Types
+// ============================================================================
+
+/**
+ * Message type for bash executions via the ! command.
+ */
+export interface BashExecutionMessage {
+	role: "bashExecution";
+	command: string;
+	output: string;
+	exitCode: number | null;
+	cancelled: boolean;
+	truncated: boolean;
+	fullOutputPath?: string;
+	timestamp: number;
+}
+
+// Extend CustomMessages via declaration merging
+declare module "@mariozechner/pi-agent-core" {
+	interface CustomMessages {
+		bashExecution: BashExecutionMessage;
+	}
+}
+
+// ============================================================================
+// Type Guards
+// ============================================================================
+
+/**
+ * Type guard for BashExecutionMessage.
+ */
+export function isBashExecutionMessage(msg: AppMessage | Message): msg is BashExecutionMessage {
+	return (msg as BashExecutionMessage).role === "bashExecution";
+}
+
+// ============================================================================
+// Message Formatting
+// ============================================================================
+
+/**
+ * Convert a BashExecutionMessage to user message text for LLM context.
+ */
+export function bashExecutionToText(msg: BashExecutionMessage): string {
+	let text = `Ran \`${msg.command}\`\n`;
+	if (msg.output) {
+		text += "```\n" + msg.output + "\n```";
+	} else {
+		text += "(no output)";
+	}
+	if (msg.cancelled) {
+		text += "\n\n(command cancelled)";
+	} else if (msg.exitCode !== null && msg.exitCode !== 0) {
+		text += `\n\nCommand exited with code ${msg.exitCode}`;
+	}
+	if (msg.truncated && msg.fullOutputPath) {
+		text += `\n\n[Output truncated. Full output: ${msg.fullOutputPath}]`;
+	}
+	return text;
+}
+
+// ============================================================================
+// Message Transformer
+// ============================================================================
+
+/**
+ * Transform AppMessages (including custom types) to LLM-compatible Messages.
+ *
+ * This is used by:
+ * - Agent's messageTransformer option (for prompt calls)
+ * - Compaction's generateSummary (for summarization)
+ */
+export function messageTransformer(messages: AppMessage[]): Message[] {
+	return messages
+		.map((m): Message | null => {
+			if (isBashExecutionMessage(m)) {
+				// Convert bash execution to user message
+				return {
+					role: "user",
+					content: [{ type: "text", text: bashExecutionToText(m) }],
+					timestamp: m.timestamp,
+				};
+			}
+			// Pass through standard LLM roles
+			if (m.role === "user" || m.role === "assistant" || m.role === "toolResult") {
+				return m as Message;
+			}
+			// Filter out unknown message types
+			return null;
+		})
+		.filter((m): m is Message => m !== null);
+}