feat(routing): decision logger via brain MCP session_log

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/routing/log.go

@@ -0,0 +1,79 @@
+package routing
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"time"
+)
+
+// LogEntry describes a single routing decision to log via the brain MCP.
+type LogEntry struct {
+	SessionID   string
+	Skill       string // the original skill the call routed (e.g., "code_review")
+	Decision    string // "local" or "claude" or "claude_fallback"
+	Message     string // free-form, e.g. "model=qwen35, pass_rate=0.94"
+	ProjectRoot string
+	DurationMs  int64
+	Failed      bool // true → final_status: "fail"; false → "skip"
+}
+
+// Logger posts session_log entries to a brain MCP at BrainURL + /mcp.
+type Logger struct {
+	BrainURL string
+	HTTP     *http.Client
+}
+
+// NewLogger creates a Logger with a 2-second HTTP timeout.
+func NewLogger(brainURL string) *Logger {
+	return &Logger{
+		BrainURL: brainURL,
+		HTTP:     &http.Client{Timeout: 2 * time.Second},
+	}
+}
+
+// LogDecision posts a session_log MCP call. Errors are returned but the caller
+// MUST NOT block real work on them — logging is best-effort.
+func (l *Logger) LogDecision(ctx context.Context, e LogEntry) error {
+	status := "skip"
+	if e.Failed {
+		status = "fail"
+	}
+	payload := map[string]any{
+		"jsonrpc": "2.0",
+		"id":      1,
+		"method":  "tools/call",
+		"params": map[string]any{
+			"name": "session_log",
+			"arguments": map[string]any{
+				"session_id":   e.SessionID,
+				"skill":        "_routing",
+				"phase":        "decide",
+				"final_status": status,
+				"message":      fmt.Sprintf("%s: %s — %s", e.Skill, e.Decision, e.Message),
+				"duration_ms":  e.DurationMs,
+				"project_root": e.ProjectRoot,
+			},
+		},
+	}
+	body, err := json.Marshal(payload)
+	if err != nil {
+		return fmt.Errorf("log: marshal: %w", err)
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, l.BrainURL+"/mcp", bytes.NewReader(body))
+	if err != nil {
+		return fmt.Errorf("log: build request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	resp, err := l.HTTP.Do(req)
+	if err != nil {
+		return fmt.Errorf("log: request: %w", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("log: server returned status %d", resp.StatusCode)
+	}
+	return nil
+}