feat: add the native silo healthcheck subcommand

Add 'silo healthcheck [live|ready|cluster|cluster-read]', a thin
anonymous HTTP client for the server's own /minio/health/* endpoints,
so containers without a shell, curl, or mc can still run health
checks. Design: silo.pgsty.com/compatibility/feature/healthcheck/

The check vocabulary maps 1:1 onto the health API paths; the probe
target is derived from the server's own --address/MINIO_ADDRESS
contract with HTTPS auto-detected from the certs directory, and can
be overridden with --url. Exit codes are 0/1 only (Docker reserves 2);
diagnostics (x-minio-server-status, quorum headers) go into a single
output line for docker inspect. The request is strictly anonymous (a
credentialed request would be rejected by the reserved-path guard),
the transport bypasses HTTP_PROXY, and certificate verification is
skipped to match kubelet HTTPS probe behavior. Cluster checks default
to a 15s deadline so the server's 10s cluster_deadline can elapse.

Compatibility notes: the preserved /minio/health/* path literals and
the MINIO_ADDRESS env var are upstream wire/config surface, reused on
purpose; the rebrand-guard baseline is regenerated for the new route
literals (tests included) with zero new exported symbols. The docker
entrypoint argv translation learns the new command name.

Verified: unit tests, entrypoint tests, go vet, plus an end-to-end
run against a live server covering all four checks, --maintenance
(412), --json, usage errors, unreachable and timeout paths.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

cmd/main.go

@@ -138,6 +138,7 @@ func newApp(name string) *cli.App {
 	// Register all commands.
 	registerCommand(serverCmd)
 	registerCommand(fmtGenCmd)
+	registerCommand(healthcheckCmd)
 
 	// Set up app.
 	cli.HelpFlag = cli.BoolFlag{