fix(release): repair pipeline defects found in v0.1.0-rc.1 rehearsal

Four defects from the rc.1 dress rehearsal:

- Dockerfile pinned golang:1.26-alpine3.23 at a 1.26.3 digest while
  go.mod requires 1.26.4; golang images set GOTOOLCHAIN=local, so the
  build hard-fails. Pin 1.26.4-alpine3.23 explicitly.
- changelog.disable swallowed --release-notes (the flag is consumed by
  the changelog pipe), publishing empty release bodies. Re-enable.
- Remove the Gitea-side GitHub relay step: redundant with
  .github/workflows/mirror-release.yml, which runs on GitHub Actions
  with the built-in token and copies the canonical Gitea assets.
- mirror-release.yml: jq '.body // empty' treats "" as truthy so the
  notes fallback never fired; use select(). Mark rc tags --prerelease.

.goreleaser.yaml

@@ -8,6 +8,7 @@ release:
   gitea:
     owner: lerkolabs
     name: uptop
+  prerelease: auto
 
 builds:
   - main: ./cmd/uptop
@@ -58,5 +59,7 @@ nfpms:
         dst: /usr/share/doc/uptop/LICENSE
         type: doc
 
-changelog:
-  disable: true
+# Changelog generation must stay enabled: the --release-notes flag is consumed
+# by the changelog pipe, so disabling it silently drops the git-cliff notes
+# (empty release body on v0.1.0-rc.1). With --release-notes set, GoReleaser
+# skips its own generation and uses the file.