feat: stream real-time progress for agent SDK install/upgrade/uninstall

Replace the spinner-only UX with live log output during agent SDK
operations, matching the existing OpenCode plugin install experience.

Backend: emit structured events (started/log/completed/failed) via
EventEmitter during npm install and binary download. npm commands now
run with piped stdio for line-by-line streaming; binary downloads
report chunked progress every 1 MB.

Frontend: subscribe to `app://agent-install` events through a new
`useAgentInstallStream` hook and render a theme-aware log terminal
below the preflight checks panel.

Also fixes the install log container in both agent settings and the
OpenCode plugins modal: auto-scroll no longer shifts the outer page,
and colours now follow the active light/dark theme.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/use-agent-install-stream.ts

@@ -0,0 +1,74 @@
+import { useCallback, useRef, useState } from "react"
+import { subscribe } from "@/lib/platform"
+import type { AgentInstallEvent, AgentInstallEventKind } from "@/lib/types"
+
+const AGENT_INSTALL_EVENT = "app://agent-install"
+
+export type AgentInstallStatus = "idle" | "running" | "success" | "failed"
+
+interface AgentInstallStreamState {
+  status: AgentInstallStatus
+  logs: string[]
+  error: string | null
+}
+
+export function useAgentInstallStream() {
+  const [state, setState] = useState<AgentInstallStreamState>({
+    status: "idle",
+    logs: [],
+    error: null,
+  })
+  const unsubRef = useRef<(() => void) | null>(null)
+
+  const start = useCallback(async (taskId: string) => {
+    setState({ status: "running", logs: [], error: null })
+
+    unsubRef.current?.()
+
+    const unsub = await subscribe<AgentInstallEvent>(
+      AGENT_INSTALL_EVENT,
+      (event) => {
+        if (event.task_id !== taskId) return
+
+        switch (event.kind as AgentInstallEventKind) {
+          case "started":
+            setState((prev) => ({ ...prev, status: "running" }))
+            break
+          case "log":
+            setState((prev) => ({
+              ...prev,
+              logs: [...prev.logs, event.payload],
+            }))
+            break
+          case "completed":
+            setState((prev) => ({
+              ...prev,
+              status: "success",
+              logs: [...prev.logs, event.payload],
+            }))
+            unsubRef.current?.()
+            break
+          case "failed":
+            setState((prev) => ({
+              ...prev,
+              status: "failed",
+              error: event.payload,
+              logs: [...prev.logs, `ERROR: ${event.payload}`],
+            }))
+            unsubRef.current?.()
+            break
+        }
+      }
+    )
+
+    unsubRef.current = unsub
+  }, [])
+
+  const reset = useCallback(() => {
+    unsubRef.current?.()
+    unsubRef.current = null
+    setState({ status: "idle", logs: [], error: null })
+  }, [])
+
+  return { ...state, start, reset }
+}