fix(rpm): drop the moved design/implementation-plan.md from %doc

The RPM %files %doc referenced design/implementation-plan.md, but the design/
docs live in the separate planning repo now — the file isn't in the public tree,
so rpmbuild failed at %files ("File not found: .../implementation-plan.md") once
the build got that far. Ship the two docs that actually exist (README.md +
packaging/README.md). Pre-existing spec rot, surfaced by the 0.8.3 release run.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/rpm/punktfunk.spec

@@ -305,7 +305,7 @@ install -Dm0644 web/web.env.example                %{buildroot}%{_datadir}/punkt
 
 %files
 %license LICENSE-MIT LICENSE-APACHE THIRD-PARTY-NOTICES.txt
-%doc README.md design/implementation-plan.md packaging/README.md
+%doc README.md packaging/README.md
 %{_bindir}/punktfunk-host
 %{_bindir}/punktfunk-tray
 %{_udevrulesdir}/60-punktfunk.rules