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/runtime/host-runtime.test.ts

@@ -134,7 +134,6 @@ function makeFetchAgentsEntry(input: {
         supportsMcpServers: true,
         supportsReasoningStream: true,
         supportsToolInvocations: true,
-        supportsTerminalMode: false,
       },
       currentModeId: null,
       availableModes: [],
@@ -1182,7 +1181,6 @@ describe("HostRuntimeStore", () => {
       const staleAgent: Agent = {
         ...stale,
         serverId: host.serverId,
-        terminal: false,
         createdAt: new Date(stale.createdAt),
         updatedAt: new Date(stale.updatedAt),
         lastUserMessageAt: null,