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/screens/workspace/workspace-draft-agent-tab.tsx

@@ -29,7 +29,6 @@ const DRAFT_CAPABILITIES: AgentCapabilityFlags = {
   supportsMcpServers: false,
   supportsReasoningStream: false,
   supportsToolInvocations: false,
-  supportsTerminalMode: false,
 };
 
 type WorkspaceDraftAgentTabProps = {
@@ -134,7 +133,6 @@ export function WorkspaceDraftAgentTab({
         serverId,
         id: tabId,
         provider: composerState.selectedProvider,
-        terminal: false,
         status: "running",
         createdAt: now,
         updatedAt: now,