Add Discord-native hybrid front-end for Jarvis (bot + bridge)

Transform isair/jarvis into a Discord-controlled voice assistant running on
the Ubuntu VNC desktop, keeping the mature ~39k-line Python brain intact.

- bot/ (Node + bun, discord.js): /자비스 slash commands (ephemeral),
  voice channel join + voice receive/playback, pluggable VNC screen broadcast
  (selfbot live / noVNC / screenshot)
- bridge/ (Python, Flask): wraps jarvis STT + run_reply_engine + Piper TTS
  behind a thin localhost HTTP API
- .env.example, scripts/ (start_bridge/start_bot/dev), README rewrite,
  docs/language-comparison.md and docs/vnc-xfce-setup.md

Language decision: hybrid (Python brain + Node/bun Discord layer) because
Discord blocks bot video; native screen broadcast only works via a Node
selfbot library.

bot/src/stream/screenshot.ts

@@ -0,0 +1,62 @@
+/**
+ * Screenshot backend (safe, no ban risk, not real-time).
+ *
+ * Periodically grabs a frame from the VNC X display with ffmpeg's x11grab and
+ * posts it to the invoking text channel. Low FPS, but works with a normal bot
+ * account and never touches Discord's selfbot surface.
+ */
+import { spawn } from "node:child_process";
+import type { AppConfig } from "../config.ts";
+import type { ScreenStreamer, StreamContext } from "./index.ts";
+
+function grabFrame(display: string, size: string): Promise<Buffer> {
+  return new Promise((resolve, reject) => {
+    const ff = spawn("ffmpeg", [
+      "-loglevel", "error",
+      "-f", "x11grab",
+      "-video_size", size,
+      "-i", display,
+      "-frames:v", "1",
+      "-f", "image2pipe",
+      "-vcodec", "png",
+      "pipe:1",
+    ]);
+    const chunks: Buffer[] = [];
+    ff.stdout.on("data", (c) => chunks.push(c));
+    ff.on("error", reject);
+    ff.on("close", (code) =>
+      code === 0 ? resolve(Buffer.concat(chunks)) : reject(new Error(`ffmpeg exited ${code}`)),
+    );
+  });
+}
+
+export class ScreenshotStreamer implements ScreenStreamer {
+  readonly kind = "screenshot" as const;
+  private timer: ReturnType<typeof setInterval> | null = null;
+  constructor(private config: AppConfig) {}
+
+  isActive() {
+    return this.timer !== null;
+  }
+
+  async start(ctx: StreamContext): Promise<string> {
+    if (!ctx.postImage) return "스크린샷을 올릴 텍스트 채널 컨텍스트가 없습니다.";
+    if (this.timer) return "이미 스크린샷 송출 중입니다.";
+    const tick = async () => {
+      try {
+        const png = await grabFrame(this.config.vncDisplay, this.config.vncResolution);
+        await ctx.postImage!(png, "vnc.png");
+      } catch (e) {
+        console.error("[screenshot] grab failed:", e);
+      }
+    };
+    this.timer = setInterval(tick, this.config.screenshotIntervalSec * 1000);
+    void tick();
+    return `📸 ${this.config.screenshotIntervalSec}초마다 VNC 스크린샷을 이 채널에 올립니다.`;
+  }
+
+  async stop(): Promise<void> {
+    if (this.timer) clearInterval(this.timer);
+    this.timer = null;
+  }
+}