refactor: remove terminal agent concept entirely

Remove the "agent can be a terminal" branching from the entire codebase.
An agent is now always a session-backed chat agent. Standalone terminal
infrastructure (terminal component, ANSI handling, terminal-stream-protocol)
is preserved.

Server: delete ManagedTerminalAgent, AgentKind, TerminalExitDetails,
launchTerminalAgent, registerTerminalAgent, handleTerminalAgentExited,
supportsTerminalMode capability, buildTerminalCreate/ResumeCommand from
all providers, terminal agent persistence/projections.

App: delete terminal-agent-panel.tsx, terminal-agent-reopen-store.ts,
terminal/terminalExit fields on agent state, "Terminal Agents" launcher
section, terminal-agent workspace setup flow, terminal badge in agent list.

CLI: remove terminal column from ls, terminal-agent error from send.

60 files changed, -3592 lines

packages/cli/src/commands/agent/send.ts

@@ -17,11 +17,6 @@ export interface AgentSendResult {
   message: string;
 }
 
-function isTerminalAgentSendError(error: unknown): boolean {
-  const message = error instanceof Error ? error.message : String(error);
-  return /terminal agents do not support structured send operations/i.test(message);
-}
-
 /** Schema for agent send output */
 export const agentSendSchema: OutputSchema<AgentSendResult> = {
   idField: "agentId",
@@ -265,15 +260,6 @@ export async function runSendCommand(
   } catch (err) {
     await client.close().catch(() => {});
 
-    if (isTerminalAgentSendError(err)) {
-      const error: CommandError = {
-        code: "TERMINAL_AGENT_UNSUPPORTED",
-        message: "Cannot send messages to terminal agents",
-        details: "Open the terminal agent from the Sessions UI and interact through its terminal.",
-      };
-      throw error;
-    }
-
     // Re-throw CommandError as-is
     if (err && typeof err === "object" && "code" in err) {
       throw err;