Add get_commands RPC for headless clients (#995)

* Add get_commands RPC for headless clients

Headless clients like Emacs can now query which commands are available.
Previously they could only discover file-based prompt templates by
scanning the filesystem; extension commands and skills were invisible.

The response includes each command's name, description, and source
(extension, template, or skill). Commands appear in the same order
as the TUI's autocomplete: extension commands first, then templates,
then skills.

Built-in TUI commands (/settings, /fork, etc.) are excluded since
they require the interactive UI. Commands like /compact have dedicated
RPC equivalents instead.

* Add location and path to get_commands response

Clients can show where commands come from (user/project/path) and
display file paths in tooltips. The data is already available on
templates and skills - just exposing it.

packages/coding-agent/src/modes/rpc/rpc-client.ts

@@ -11,7 +11,7 @@ import type { ImageContent } from "@mariozechner/pi-ai";
 import type { SessionStats } from "../../core/agent-session.js";
 import type { BashResult } from "../../core/bash-executor.js";
 import type { CompactionResult } from "../../core/compaction/index.js";
-import type { RpcCommand, RpcResponse, RpcSessionState } from "./rpc-types.js";
+import type { RpcCommand, RpcResponse, RpcSessionState, RpcSlashCommand } from "./rpc-types.js";
 
 // ============================================================================
 // Types
@@ -370,6 +370,14 @@ export class RpcClient {
 		return this.getData<{ messages: AgentMessage[] }>(response).messages;
 	}
 
+	/**
+	 * Get available commands (extension commands, prompt templates, skills).
+	 */
+	async getCommands(): Promise<RpcSlashCommand[]> {
+		const response = await this.send({ type: "get_commands" });
+		return this.getData<{ commands: RpcSlashCommand[] }>(response).commands;
+	}
+
 	// =========================================================================
 	// Helpers
 	// =========================================================================

packages/coding-agent/src/modes/rpc/rpc-mode.ts

@@ -26,6 +26,7 @@ import type {
 	RpcExtensionUIResponse,
 	RpcResponse,
 	RpcSessionState,
+	RpcSlashCommand,
 } from "./rpc-types.js";
 
 // Re-export types for consumers
@@ -497,6 +498,48 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 				return success(id, "get_messages", { messages: session.messages });
 			}
 
+			// =================================================================
+			// Commands (available for invocation via prompt)
+			// =================================================================
+
+			case "get_commands": {
+				const commands: RpcSlashCommand[] = [];
+
+				// Extension commands
+				for (const { command, extensionPath } of session.extensionRunner?.getRegisteredCommandsWithPaths() ?? []) {
+					commands.push({
+						name: command.name,
+						description: command.description,
+						source: "extension",
+						path: extensionPath,
+					});
+				}
+
+				// Prompt templates (source is always "user" | "project" | "path" in coding-agent)
+				for (const template of session.promptTemplates) {
+					commands.push({
+						name: template.name,
+						description: template.description,
+						source: "template",
+						location: template.source as RpcSlashCommand["location"],
+						path: template.filePath,
+					});
+				}
+
+				// Skills (source is always "user" | "project" | "path" in coding-agent)
+				for (const skill of session.resourceLoader.getSkills().skills) {
+					commands.push({
+						name: `skill:${skill.name}`,
+						description: skill.description,
+						source: "skill",
+						location: skill.source as RpcSlashCommand["location"],
+						path: skill.filePath,
+					});
+				}
+
+				return success(id, "get_commands", { commands });
+			}
+
 			default: {
 				const unknownCommand = command as { type: string };
 				return error(undefined, unknownCommand.type, `Unknown command: ${unknownCommand.type}`);

packages/coding-agent/src/modes/rpc/rpc-types.ts

@@ -60,7 +60,28 @@ export type RpcCommand =
 	| { id?: string; type: "get_last_assistant_text" }
 
 	// Messages
-	| { id?: string; type: "get_messages" };
+	| { id?: string; type: "get_messages" }
+
+	// Commands (available for invocation via prompt)
+	| { id?: string; type: "get_commands" };
+
+// ============================================================================
+// RPC Slash Command (for get_commands response)
+// ============================================================================
+
+/** A command available for invocation via prompt */
+export interface RpcSlashCommand {
+	/** Command name (without leading slash) */
+	name: string;
+	/** Human-readable description */
+	description?: string;
+	/** What kind of command this is */
+	source: "extension" | "template" | "skill";
+	/** Where the command was loaded from (undefined for extensions) */
+	location?: "user" | "project" | "path";
+	/** File path to the command source */
+	path?: string;
+}
 
 // ============================================================================
 // RPC State
@@ -168,6 +189,15 @@ export type RpcResponse =
 	// Messages
 	| { id?: string; type: "response"; command: "get_messages"; success: true; data: { messages: AgentMessage[] } }
 
+	// Commands
+	| {
+			id?: string;
+			type: "response";
+			command: "get_commands";
+			success: true;
+			data: { commands: RpcSlashCommand[] };
+	  }
+
 	// Error response (any command can fail)
 	| { id?: string; type: "response"; command: string; success: false; error: string };