feat(ai): backfill context on join + token-budget window

The server already ships the full RAM message backlog in the init frame; the
agent was discarding it. _seed_transcript now decrypts that history with the
room key (skipping our own lines, control frames, and undecryptable blobs) so
the agent has context the moment it joins instead of starting amnesiac.

_window() replaces the fixed last-12 slice on both the answer and sandbox
paths: it walks newest-to-oldest and keeps messages up to --token-budget
(approx, ~4 chars/token), still capped at --context-window count. Keeps small
local models inside their effective context. Nothing touches disk.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd_chat/agent/__main__.py

@@ -76,7 +76,10 @@ def main() -> None:
     ap.add_argument("--model", default=None, help="model name (provider default if omitted)")
     ap.add_argument("--base-url", default=None, help="endpoint for openai-compatible providers")
     ap.add_argument("--system", default=None, help="override the system prompt")
-    ap.add_argument("--context-window", type=int, default=12)
+    ap.add_argument("--context-window", type=int, default=12,
+                    help="max prior messages fed to the model per reply")
+    ap.add_argument("--token-budget", type=int, default=3000,
+                    help="approx token cap on the context window (whichever is smaller wins)")
     ap.add_argument("--list-models", action="store_true",
                     help="list models the backend can serve, then exit")
     ap.add_argument("--check", action="store_true",
@@ -113,6 +116,7 @@ def main() -> None:
         args.server, args.port, name=args.name, provider=provider,
         password=args.password, insecure=args.insecure, no_tls=args.no_tls,
         system_prompt=args.system, context_window=args.context_window,
+        token_budget=args.token_budget,
     )
     try:
         bridge.run()