feat(coding-agent): add set_session_name RPC command (#1075)

- Add set_session_name command with empty name validation
- Expose sessionName in get_state response
- Add setSessionName() to AgentSession and RpcClient
- Document in docs/rpc.md

packages/coding-agent/src/core/agent-session.ts

@@ -590,6 +590,11 @@ export class AgentSession {
 		return this.sessionManager.getSessionId();
 	}
 
+	/** Current session display name, if set */
+	get sessionName(): string | undefined {
+		return this.sessionManager.getSessionName();
+	}
+
 	/** Scoped models for cycling (from --models flag) */
 	get scopedModels(): ReadonlyArray<{ model: Model<any>; thinkingLevel: ThinkingLevel }> {
 		return this._scopedModels;
@@ -2181,6 +2186,13 @@ export class AgentSession {
 		return true;
 	}
 
+	/**
+	 * Set a display name for the current session.
+	 */
+	setSessionName(name: string): void {
+		this.sessionManager.appendSessionInfo(name);
+	}
+
 	/**
 	 * Create a fork from a specific entry.
 	 * Emits before_fork/fork session events to extensions.