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>

cmd/globals.go

@@ -176,13 +176,13 @@ var (
 	// Global user opts context
 	globalServerCtxt serverCtxt
 
-	// Indicates if the running minio server is distributed setup.
+	// Indicates whether the running Silo server is a distributed setup.
 	globalIsDistErasure = false
 
-	// Indicates if the running minio server is an erasure-code backend.
+	// Indicates whether the running Silo server is an erasure-code backend.
 	globalIsErasure = false
 
-	// Indicates if the running minio server is in single drive XL mode.
+	// Indicates whether the running Silo server is in single-drive XL mode.
 	globalIsErasureSD = false
 
 	// Indicates if server code should go through testing path.
@@ -201,8 +201,8 @@ var (
 	// globalBrowserConfig Browser user configurable settings
 	globalBrowserConfig browser.Config
 
-	// This flag is set to 'true' when MINIO_UPDATE env is set to 'off'. Default is false.
-	globalInplaceUpdateDisabled = false
+	// Silo permanently disables inherited in-place updates.
+	globalInplaceUpdateDisabled = true
 
 	// Captures site name and region
 	globalSite config.Site
@@ -433,10 +433,6 @@ var (
 	// MinIO client
 	globalMinioClient *minio.Client
 
-	// Public key for subnet confidential information
-	subnetAdminPublicKey    = []byte("-----BEGIN PUBLIC KEY-----\nMIIBCgKCAQEAyC+ol5v0FP+QcsR6d1KypR/063FInmNEFsFzbEwlHQyEQN3O7kNI\nwVDN1vqp1wDmJYmv4VZGRGzfFw1q+QV7K1TnysrEjrqpVxfxzDQCoUadAp8IxLLc\ns2fjyDNxnZjoC6fTID9C0khKnEa5fPZZc3Ihci9SiCGkPmyUyCGVSxWXIKqL2Lrj\nyDc0pGeEhWeEPqw6q8X2jvTC246tlzqpDeNsPbcv2KblXRcKniQNbBrizT37CKHQ\nM6hc9kugrZbFuo8U5/4RQvZPJnx/DVjLDyoKo2uzuVQs4s+iBrA5sSSLp8rPED/3\n6DgWw3e244Dxtrg972dIT1IOqgn7KUJzVQIDAQAB\n-----END PUBLIC KEY-----")
-	subnetAdminPublicKeyDev = []byte("-----BEGIN PUBLIC KEY-----\nMIIBCgKCAQEArhQYXQd6zI4uagtVfthAPOt6i4AYHnEWCoNeAovM4MNl42I9uQFh\n3VHkbWj9Gpx9ghf6PgRgK+8FcFvy+StmGcXpDCiFywXX24uNhcZjscX1C4Esk0BW\nidfI2eXYkOlymD4lcK70SVgJvC693Qa7Z3FE1KU8Nfv2bkxEE4bzOkojX9t6a3+J\nR8X6Z2U8EMlH1qxJPgiPogELhWP0qf2Lq7GwSAflo1Tj/ytxvD12WrnE0Rrj/8yP\nSnp7TbYm91KocKMExlmvx3l2XPLxeU8nf9U0U+KOmorejD3MDMEPF+tlk9LB3JWP\nZqYYe38rfALVTn4RVJriUcNOoEpEyC0WEwIDAQAB\n-----END PUBLIC KEY-----")
-
 	// dynamic sleeper to avoid thundering herd for trash folder expunge routine
 	deleteCleanupSleeper = newDynamicSleeper(5, 25*time.Millisecond, false)