fx: disable T3 Code telemetry by default

T3 Code sends telemetry to PostHog unless T3CODE_TELEMETRY_ENABLED is set
to false (it defaults to true). Set it in the fx image so workspaces opt
out by default.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Eszmn7WakQXem76vE9g7ZL

Author: claude

fx/Dockerfile

@@ -13,6 +13,9 @@ ENV PATH=$GOROOT/bin:$GOPATH/bin:$PATH
 ENV XDG_RUNTIME_DIR=/run/user/1000
 ENV DOCKER_HOST=unix:///run/user/1000/docker.sock
 
+# T3 Code: opt out of telemetry by default
+ENV T3CODE_TELEMETRY_ENABLED=false
+
 USER root
 
 # Helper to bring up the rootless Docker daemon at workspace start