fix(routing): repair pass-rate instrumentation (3 bugs) — #73, #35

The #35 data gate could never fill: a real review call routed cleanly to
qwen36 but /pass-rate stayed total:0 under every key. Root cause was three
independent defects in the session_log path, each alone fatal:

- A: a successful routed call logged final_status "skip", never "pass".
  /pass-rate computes pass/(pass+fail) and skips count as neither, so the
  >=0.90 gate was mathematically unreachable. Success now logs "pass".
- B: every record was written under skill "_routing", so /pass-rate?skill=
  review|debug (what #35 measures) always read zero. Now uses the real
  e.Skill; routing decisions stay groupable via session_id "_routing".
- C: the session_log POST to the bearer-gated ingestion /mcp carried no
  Authorization header → silent 401, swallowed by best-effort logging
  (the documented mcpclient-empty-token-silent-401 footgun). Logger now
  takes a token (BRAIN_MCP_TOKEN) and sets the bearer when non-empty.

Tests rewritten to assert correct behavior (they had encoded the bugs:
"skip" on success, "_routing" skill). New test covers the auth header and
the empty-token path.

Infra (BRAIN_MCP_TOKEN ExternalSecret + env on the routing deployment) and
redeploy follow separately. Refs #73, #35.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/routing/router_test.go

@@ -50,7 +50,7 @@ func newRouter(t *testing.T, llm *fakeLLM, passRate float64) (*routing.Router, *
 
 	r := &routing.Router{
 		Fetcher:       routing.NewFetcher(brain.URL, "7d", time.Minute),
-		Logger:        routing.NewLogger(brain.URL),
+		Logger:        routing.NewLogger(brain.URL, ""),
 		Policy:        routing.Policy{Floor: 0.9, Ceil: 0.7},
 		FastModel:     "koala/qwen35-9b-fast",
 		ThinkingModel: "iguana/gemma4-26b",
@@ -117,7 +117,7 @@ func TestRouterDefaultsToFastWhenBrainUnreachable(t *testing.T) {
 	llm := &fakeLLM{resp: "ok"}
 	r := &routing.Router{
 		Fetcher:       routing.NewFetcher(brain.URL, "7d", time.Minute),
-		Logger:        routing.NewLogger(brain.URL),
+		Logger:        routing.NewLogger(brain.URL, ""),
 		Policy:        routing.Policy{Floor: 0.9, Ceil: 0.7},
 		FastModel:     "koala/qwen35-9b-fast",
 		ThinkingModel: "iguana/gemma4-26b",