release: 1.9.5: GHCR release fix: keep per-arch images, sweep orphaned versions (CI only)

CI-only cut. The release workflow deleted the X.Y.Z-amd64 / X.Y.Z-arm64
package versions after the manifest merge; those versions are the images the
X.Y.Z manifest points at, so pulling 1.3.0 failed with a missing manifest.
No product code changes.

Fixed:
  - release.yml: the per-arch tags stay in GHCR. GHCR cannot drop a tag
    without deleting the image behind it.
  - release.yml: the merge job now removes only untagged package versions
    that no tagged manifest references (leftovers of publishing a version
    again); any failed lookup aborts the step before anything is deleted.
  - development.md release description and version table, image pin 1.9.5
    in compose, README, guide.

Full detail: CHANGELOG.md (1.9.5).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -112,7 +112,7 @@ docker run --rm -d --name selfpost-try \
   -e SELFPOST_HOSTNAME=mail.local.test \
   -e PANEL_COOKIE_SECURE=false \
   -v selfpost-try-data:/data \
-  ghcr.io/mixeme/selfpost:1.9.4
+  ghcr.io/mixeme/selfpost:1.9.5
 ```
 
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