feat(cursor): implement StatusError for conductor cooldown integration

Cursor executor errors were plain fmt.Errorf — the conductor couldn't
extract HTTP status codes, so exhausted accounts never entered cooldown.

Changes:
- Add ConnectError struct to proto/connect.go: ParseConnectEndStream now
  returns *ConnectError with Code/Message fields for precise matching
- Add cursorStatusErr implementing StatusError + RetryAfter interfaces
- Add classifyCursorError() with two-layer classification:
  Layer 1: exact match on ConnectError.Code (gRPC standard codes)
    resource_exhausted → 429, unauthenticated → 401,
    permission_denied → 403, unavailable → 503, internal → 500
  Layer 2: fuzzy string match for H2 errors (RST_STREAM → 502)
- Log all ConnectError code/message pairs for observing real server
  error codes (we have no samples yet)
- Wrap Execute and ExecuteStream error returns with classifyCursorError

Now the conductor properly marks Cursor auths as cooldown on quota errors,
enabling exponential backoff and round-robin failover.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/auth/cursor/proto/connect.go

@@ -41,8 +41,21 @@ func ParseConnectFrame(buf []byte) (flags byte, payload []byte, consumed int, ok
 	return flags, buf[5:total], total, true
 }
 
+// ConnectError is a structured error from the Connect protocol end-of-stream trailer.
+// The Code field contains the server-defined error code (e.g. gRPC standard codes
+// like "resource_exhausted", "unauthenticated", "permission_denied", "unavailable").
+type ConnectError struct {
+	Code    string // server-defined error code
+	Message string // human-readable error description
+}
+
+func (e *ConnectError) Error() string {
+	return fmt.Sprintf("Connect error %s: %s", e.Code, e.Message)
+}
+
 // ParseConnectEndStream parses a Connect end-of-stream frame payload (JSON).
 // Returns nil if there is no error in the trailer.
+// On error, returns a *ConnectError with the server's error code and message.
 func ParseConnectEndStream(data []byte) error {
 	if len(data) == 0 {
 		return nil
@@ -65,7 +78,7 @@ func ParseConnectEndStream(data []byte) error {
 		if msg == "" {
 			msg = "Unknown error"
 		}
-		return fmt.Errorf("Connect error %s: %s", code, msg)
+		return &ConnectError{Code: code, Message: msg}
 	}
 	return nil
 }