Stabilize runtime image, add PR-based version gating, drop arm/v7

Dockerfile:
- Keep build stage on debian:sid-slim (required for riscv64 Go support)
- Switch runtime stage to debian:bookworm-slim for stable, predictable
  package names — eliminates the libcbor0 class of breakage for users

update-check.py:
- Create a branch and open a PR instead of pushing directly to master
- PR body links to upstream release notes and prompts review of new
  dependencies before merge
- Remove dead deb/PACKAGE code

build.yaml:
- Drop linux/arm/v7 — upstream go-libfido2 is incompatible with 32-bit
  ARM address space as of v3.22.0; not fixable without upstream changes
- Add VERSION to pull_request trigger paths so the test job builds
  and validates every version bump PR before it can be merged

update-check.yaml:
- Pass GITHUB_TOKEN and GITHUB_REPOSITORY to script for PR creation

README.md:
- Document arm/v7 as unsupported with reason

.github/workflows/update-check.yaml

@@ -24,4 +24,7 @@ jobs:
     - name: Install dependencies
       run: pip install requests
     - name: Check Update
+      env:
+        GITHUB_TOKEN: ${{ secrets.PERSONAL_TOKEN }}
+        GITHUB_REPOSITORY: ${{ github.repository }}
       run: python3 update-check.py ${{ github.event_name == 'pull_request' }}