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/bucket-handlers.go

@@ -1415,7 +1415,7 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 						Object:       ObjectInfo{Name: objInfo.Name},
 						ReqParams:    extractReqParams(r),
 						RespElements: extractRespElements(w),
-						UserAgent:    fmt.Sprintf("%s MinIO-Fan-Out (failed: %v)", r.UserAgent(), errs[i]),
+						UserAgent:    fmt.Sprintf("%s Silo-Fan-Out (failed: %v)", r.UserAgent(), errs[i]),
 						Host:         handlers.GetSourceIP(r),
 					})
 					continue
@@ -1434,7 +1434,7 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 					Object:       objInfo,
 					ReqParams:    extractReqParams(r),
 					RespElements: extractRespElements(w),
-					UserAgent:    r.UserAgent() + " " + "MinIO-Fan-Out",
+					UserAgent:    r.UserAgent() + " " + "Silo-Fan-Out",
 					Host:         handlers.GetSourceIP(r),
 				})
 			}
@@ -1462,7 +1462,7 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 					Object:       eventArgsList[i].Object,
 					ReqParams:    extractReqParams(r),
 					RespElements: extractRespElements(w),
-					UserAgent:    r.UserAgent() + " " + "MinIO-Fan-Out",
+					UserAgent:    r.UserAgent() + " " + "Silo-Fan-Out",
 					Host:         handlers.GetSourceIP(r),
 				})
 
@@ -1757,7 +1757,7 @@ func (api objectAPIHandlers) DeleteBucketHandler(w http.ResponseWriter, r *http.
 
 	if globalDNSConfig != nil {
 		if err := globalDNSConfig.Delete(bucket); err != nil {
-			dnsLogIf(ctx, fmt.Errorf("Unable to delete bucket DNS entry %w, please delete it manually, bucket on MinIO no longer exists", err))
+			dnsLogIf(ctx, fmt.Errorf("Unable to delete bucket DNS entry %w; please delete it manually because the bucket no longer exists on Silo", err))
 			writeErrorResponse(ctx, w, toAPIError(ctx, err), r.URL)
 			return
 		}