feat(server): present Silo identity and close the inherited upstream services

Two coupled changes that must land together, because the same files carry both:
the server now identifies itself as Silo, and every path that would have called
home to a MinIO-operated service is closed.

Product identity
- build-constants.go: store name, UA name and startup banner become Silo. The
  Go identifiers (MinioStoreName, MinioBannerName, ...) keep their names on
  purpose - renaming exported symbols would churn the compatibility surface for
  a cosmetic gain, and the rebrand guard freezes that surface.
- main.go, server-startup-msg.go, ftp-server.go and the user-visible log, help
  and error strings across cmd/ and internal/ switch to Silo. Original MinIO
  copyright, LICENSE, NOTICE and CREDITS are untouched; --version now prints
  the upstream copyright, the pgsty modification notice, and the trademark
  policy's approved "based on MinIO technology" attribution.
- api-headers.go: the HTTP Server header becomes "Silo". This is the one
  externally observable identity change, so TestCommonHeadersUseSiloProductName
  pins it - probes that sniff for "MinIO" must move to capability detection.
- Prometheus metric HELP strings keep their MinIO wording. They are part of the
  metrics contract the guard protects, not product copy.

Configuration directory
- config-dir.go: new installs use ~/.silo. If only ~/.minio exists it is still
  read, with a one-time notice and no files moved. If both exist ~/.silo wins
  and an ambiguity warning is emitted; an explicit --config-dir always wins.
  Covered by TestSelectDefaultConfigDir. The internal .minio.sys layout is
  never renamed - this rule applies to the user config directory only.

Upstream service lockdown
- globalInplaceUpdateDisabled is now true at initialization rather than being
  set from MINIO_UPDATE. common-main.go still parses MINIO_UPDATE so upgrading
  nodes do not fail on an unknown key, but warns that the value is ignored;
  there is no way to re-enable the updater. TestInplaceUpdateCannotBeEnabled
  guards that. Without this, an admin with mc could have overwritten
  /usr/bin/silo with an upstream MinIO binary.
- verifyBinary and commitBinary refuse early; the ServerUpdate v1/v2 admin
  routes and the peer-rest update endpoints stay registered and keep returning
  the existing programmatic error, so clients see a stable failure rather than
  a 404.
- MinioReleaseBaseURL and defaultMinisignPubkey are emptied: no dl.min.io
  download root, and upstream's minisign key is no longer a trust root for
  anything this fork ships.
- cmd/callhome.go is deleted and internal/config/subnet/ is reduced to parsing
  its old keys and reporting that the integration is disabled. config-current.go
  warns instead of failing when callhome or SUBNET settings are present, so an
  upgraded node with those keys still starts.
- internal/config/errors.go replaces the MinIO Slack and support entry points
  with Silo documentation and issue links. Error codes and programmatic fields
  are unchanged.

Verified: the compatibility baseline is unchanged except for the deliberate
removal of the /api/health/upload SUBNET route; go build, go vet and the full
cmd/ and internal/ unit suites pass; a locally built binary starts, serves
S3/Admin/metrics on the unchanged /minio/* routes, answers with Server: Silo,
and falls back to a pre-existing ~/.minio with the expected notice.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/kms/config.go

@@ -358,11 +358,11 @@ func IsPresent() (bool, error) {
 
 	switch {
 	case kmsPresent && kesPresent:
-		return false, errors.New("kms: configuration for MinIO KMS and MinIO KES is present")
+		return false, errors.New("kms: both KMS and KES configuration are present")
 	case kmsPresent && staticKeyPresent:
-		return false, errors.New("kms: configuration for MinIO KMS and static KMS key is present")
+		return false, errors.New("kms: both KMS and static-key configuration are present")
 	case kesPresent && staticKeyPresent:
-		return false, errors.New("kms: configuration for MinIO KES and static KMS key is present")
+		return false, errors.New("kms: both KES and static-key configuration are present")
 	}
 
 	// Next, we check that all required configuration for the concrete
@@ -375,16 +375,16 @@ func IsPresent() (bool, error) {
 		return false, nil // No KMS config present
 	case kmsPresent:
 		if !isPresent(EnvKMSEndpoint) {
-			return false, fmt.Errorf("kms: incomplete configuration for MinIO KMS: missing '%s'", EnvKMSEndpoint)
+			return false, fmt.Errorf("kms: incomplete KMS configuration: missing '%s'", EnvKMSEndpoint)
 		}
 		if !isPresent(EnvKMSEnclave) {
-			return false, fmt.Errorf("kms: incomplete configuration for MinIO KMS: missing '%s'", EnvKMSEnclave)
+			return false, fmt.Errorf("kms: incomplete KMS configuration: missing '%s'", EnvKMSEnclave)
 		}
 		if !isPresent(EnvKMSDefaultKey) {
-			return false, fmt.Errorf("kms: incomplete configuration for MinIO KMS: missing '%s'", EnvKMSDefaultKey)
+			return false, fmt.Errorf("kms: incomplete KMS configuration: missing '%s'", EnvKMSDefaultKey)
 		}
 		if !isPresent(EnvKMSAPIKey) {
-			return false, fmt.Errorf("kms: incomplete configuration for MinIO KMS: missing '%s'", EnvKMSAPIKey)
+			return false, fmt.Errorf("kms: incomplete KMS configuration: missing '%s'", EnvKMSAPIKey)
 		}
 		return true, nil
 	case staticKeyPresent:
@@ -394,24 +394,24 @@ func IsPresent() (bool, error) {
 		return true, nil
 	case kesPresent:
 		if !isPresent(EnvKESEndpoint) {
-			return false, fmt.Errorf("kms: incomplete configuration for MinIO KES: missing '%s'", EnvKESEndpoint)
+			return false, fmt.Errorf("kms: incomplete KES configuration: missing '%s'", EnvKESEndpoint)
 		}
 		if !isPresent(EnvKESDefaultKey) {
-			return false, fmt.Errorf("kms: incomplete configuration for MinIO KES: missing '%s'", EnvKESDefaultKey)
+			return false, fmt.Errorf("kms: incomplete KES configuration: missing '%s'", EnvKESDefaultKey)
 		}
 
 		if isPresent(EnvKESClientKey, EnvKESClientCert, EnvKESClientPassword) {
 			if isPresent(EnvKESAPIKey) {
-				return false, fmt.Errorf("kms: invalid configuration for MinIO KES: '%s' and client certificate is present", EnvKESAPIKey)
+				return false, fmt.Errorf("kms: invalid KES configuration: '%s' and client certificate are both present", EnvKESAPIKey)
 			}
 			if !isPresent(EnvKESClientCert) {
-				return false, fmt.Errorf("kms: incomplete configuration for MinIO KES: missing '%s'", EnvKESClientCert)
+				return false, fmt.Errorf("kms: incomplete KES configuration: missing '%s'", EnvKESClientCert)
 			}
 			if !isPresent(EnvKESClientKey) {
-				return false, fmt.Errorf("kms: incomplete configuration for MinIO KES: missing '%s'", EnvKESClientKey)
+				return false, fmt.Errorf("kms: incomplete KES configuration: missing '%s'", EnvKESClientKey)
 			}
 		} else if !isPresent(EnvKESAPIKey) {
-			return false, errors.New("kms: incomplete configuration for MinIO KES: missing authentication method")
+			return false, errors.New("kms: incomplete KES configuration: missing authentication method")
 		}
 		return true, nil
 	}