docs: update standalone LS, MITM, and panel stream investigation

- Add panel-stream-investigation.md documenting dead end
- Update KNOWN_ISSUES: move polling and panel stream to resolved
- Update GEMINI.md with standalone LS section and new MITM setup
- Fix standalone-ls-todo to reflect default mode

docs/standalone-ls-todo.md

@@ -2,7 +2,7 @@
 
 ## Status: ✅ FULLY IMPLEMENTED (incl. MITM interception)
 
-The standalone LS is fully working via `--standalone` flag on the proxy.
+The standalone LS is the default mode. Disable with `--no-standalone`.
 All cascade types (sync, streaming, multi-turn) and all endpoints work.
 MITM interception captures real token usage from Google's API.
 
@@ -38,7 +38,7 @@ When `scripts/mitm-redirect.sh install` has been run:
 sudo ./scripts/mitm-redirect.sh install
 
 # Run
-RUST_LOG=info ./target/release/antigravity-proxy --standalone
+RUST_LOG=info ./target/release/antigravity-proxy
 
 # Check intercepted usage
 curl -s http://localhost:8741/v1/usage | jq .