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/contexts/session-status-tracking.test.ts

@@ -7,7 +7,6 @@ function createAgent(status: Agent["status"]): Agent {
     serverId: "server-1",
     id: "agent-1",
     provider: "codex",
-    terminal: false,
     status,
     createdAt: new Date(0),
     updatedAt: new Date(0),
@@ -20,7 +19,6 @@ function createAgent(status: Agent["status"]): Agent {
       supportsMcpServers: true,
       supportsReasoningStream: true,
       supportsToolInvocations: true,
-      supportsTerminalMode: false,
     },
     currentModeId: null,
     availableModes: [],