feat(M40): F5 BIG-IP target connector via iControl REST

Replace 190-line stub with full iControl REST implementation (~580 lines).
Token auth with 401 auto-retry, file upload + crypto object install,
transaction-based atomic SSL profile updates, cleanup on failure.
Injectable F5Client interface for cross-platform testing. 32 tests.

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

cmd/agent/main.go

@@ -585,7 +585,11 @@ func (a *Agent) createTargetConnector(targetType string, configJSON json.RawMess
 				return nil, fmt.Errorf("invalid F5 config: %w", err)
 			}
 		}
-		return f5.New(&cfg, a.logger), nil
+		conn, err := f5.New(&cfg, a.logger)
+		if err != nil {
+			return nil, fmt.Errorf("failed to create F5 connector: %w", err)
+		}
+		return conn, nil
 
 	case "IIS":
 		var cfg iis.Config