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/app/src/hooks/use-agent-screen-state-machine.ts

@@ -6,13 +6,6 @@ export interface AgentScreenAgent {
   status: "initializing" | "idle" | "running" | "error" | "closed";
   cwd: string;
   lastError?: string | null;
-  terminalExit?: {
-    command: string;
-    message: string;
-    exitCode: number | null;
-    signal: number | null;
-    outputLines: string[];
-  } | null;
   projectPlacement?: {
     checkout?: {
       cwd?: string;