fix(combo): keep nested combos as single units and stop 0-token detail rows

Nested combos (comboA lists comboB, comboC, …) now stay one slot each:
the inner combo always runs as fallback to produce a single answer.
Failed hops are no longer written to Details/usage, and streaming no
longer inserts a 0-token placeholder row.

- chat.js: comboStack cycle detection; nested combos forced to fallback;
  persistUsage="success-only" for combo hops
- combo.js: discardResponse() cancels unused bodies (fusion timeout /
  fallback) so dropped streams fire onStreamComplete; getComboModelsFromData
  keeps nested names and honors enabled=false
- requestDetail.js: tokensForDetail() canonicalizes Claude/Gemini usage;
  shouldPersistRequestDetail() skips streaming-start and non-success hops
- streamingHandler.js: drop the 0-token streaming placeholder write
- RequestDetailsTab.js: read Gemini/Claude token names; show "streaming"
  status in amber
- tests: add combo-nested.test.js (13 cases)
- gitignore: ignore local .vitest/ artifacts

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

.gitignore

@@ -94,4 +94,6 @@ graphify-out/*
 .commandcode/
 
 # Pi subagent run artifacts
-.pi-subagents/
+.pi-subagents/
+# Vitest local artifacts
+.vitest/