refactor: simplify tool server to asyncio tasks with per-agent isolation

- Replace multiprocessing/threading with single asyncio task per agent
- Add task cancellation: new request cancels previous for same agent
- Add per-agent state isolation via ContextVar for Terminal, Browser, Python managers
- Add posthog telemetry for tool execution errors (timeout, http, sandbox)
- Fix proxy manager singleton pattern
- Increase client timeout buffer over server timeout
- Add context.py to Dockerfile

strix/tools/proxy/proxy_manager.py

@@ -785,6 +785,7 @@ _PROXY_MANAGER: ProxyManager | None = None
 
 
 def get_proxy_manager() -> ProxyManager:
+    global _PROXY_MANAGER  # noqa: PLW0603
     if _PROXY_MANAGER is None:
-        return ProxyManager()
+        _PROXY_MANAGER = ProxyManager()
     return _PROXY_MANAGER