feat(agent): global bandwidth cap — the agent can't saturate the desktop's network

One shared TokenBucket (default 8 MB/s; BANDWIDTH_LIMIT_MB_S, 0 = unlimited;
live MB/s dial + net readout in the control UI) is charged by every still
download (streamed chunk reads) and every ffmpeg video stream (metered from
outside via /proc/<pid>/io and SIGSTOP/SIGCONTed into budget).

Why: D1 re-measurement 2026-07-02 — the idle link moves ~38 MB/s, but 8
unthrottled downloaders bufferbloated it to ~1-1.5 MB/s PER STREAM (operator's
browser included). Capping the aggregate keeps the desktop usable and still
beats the collapsed sweep throughput it replaces. Agent build 2026-07-02.4.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

agent/fc_agent/config.py

@@ -48,6 +48,9 @@ class Config:
     #                             higher keeps more frames, 0 disables
     ffmpeg_timeout: float  # hard ceiling (s) for ffmpeg-from-URL video sampling;
     #                        generous so a SLOW media link still completes
+    bandwidth_limit_mb_s: float  # aggregate download cap in MEGABYTES/s across
+    #                              all downloaders + video streams (0 = unlimited);
+    #                              tunable live from the agent UI
 
     @classmethod
     def from_env(cls) -> Config:
@@ -77,4 +80,11 @@ class Config:
             dedupe_iou=float(os.environ.get("DEDUPE_IOU", "0.85")),
             frame_dedupe_distance=int(os.environ.get("FRAME_DEDUPE_DISTANCE", "8")),
             ffmpeg_timeout=float(os.environ.get("FFMPEG_TIMEOUT", "1200")),
+            # Default 8 MB/s (~64 Mbit/s): ~20% of the measured ~300 Mbit/s home
+            # WiFi, so browsing stays snappy while the agent works — yet MORE
+            # sweep throughput than the self-inflicted congestion collapse this
+            # replaces (2026-07-02: 8 unthrottled downloaders bufferbloated the
+            # link to ~1-1.5 MB/s per stream, browser included). Raise it (or 0)
+            # from the agent UI on wired/faster networks.
+            bandwidth_limit_mb_s=float(os.environ.get("BANDWIDTH_LIMIT_MB_S", "8")),
         )