docs: adopt the no-CLA plus DCO policy and fix copyright terms

Brings the server in line with the contribution and copyright policy
already adopted in pgsty/mc.

Copyright terms were derived from the clock: startupBanner overwrote the
ldflags-injected CopyrightYear with time.Now().Year() and printed it as
the end of MinIO, Inc.'s term, so every January would have extended the
upstream copyright claim past the 2025 end of upstream development, and
release builds silently discarded the injected year. Both banners now
credit MinIO, Inc. for 2015-2025 and PGSTY from 2025 through the
release-stamped year, falling back to the current year in source builds.
NOTICE names PGSTY as the holder of the fork's modifications, matching
the banners and the packaging vendor field.

Contributions are accepted inbound=outbound under AGPL-3.0-or-later with
no CLA - the core is Copyright MinIO, Inc., so the combined work can
never be relicensed and a CLA would buy nothing. What a fork carrying a
downstream delta does need is provenance, so DCO 1.1 sign-off is now
mandatory and enforced by a workflow that rejects unsigned non-bot
commits. CONTRIBUTING documents sign-off, repair, cherry-pick provenance,
dual copyright headers, trailer preservation across squash merges, and
that assistive-tooling trailers carry no authorship or copyright claim.

verify-rebrand.sh pins the copyright split and the policy files so
neither can regress.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Feng Ruohang <rh@vonng.com>

NOTICE

@@ -8,7 +8,7 @@ separate copyright notices and license terms. Your use of the source
 code for these subcomponents is subject to the terms and conditions
 of GNU Affero General Public License 3.0.
 
-Silo Project modifications, (C) 2025-2026 The Silo Authors.
+Silo Project modifications, (C) 2025-2026 PGSTY.
 
 Silo is an independent community-maintained project incorporating MinIO
 source code. It is not affiliated with or endorsed by MinIO, Inc. Modified