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

README.md

@@ -30,7 +30,7 @@ Images are built for the following platforms from source:
 |---|---|
 | `linux/amd64` | Yes |
 | `linux/arm64/v8` | Yes |
-| `linux/arm/v7` | Yes |
+| `linux/arm/v7` | No — upstream go-libfido2 dependency does not support 32-bit ARM as of v3.22.0 |
 | `linux/riscv64` | Yes |
 
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