Restore Node.js to runner image for JavaScript Actions (#163)

## Summary

- Restores Node.js 20 LTS to the Forgejo runner job image
- `actions/checkout@v4` and other JavaScript Actions require `node` in the job container
- The Phase 3 simplification (PR #162) accidentally removed it, breaking all CI runs

## Changes

- `containers/forgejo-runner/Dockerfile`: Add `gnupg` (for nodesource GPG key) and Node.js 20 via nodesource
- Changelog fragment

## Test plan

- [ ] Merge, release as `forgejo-runner-v3.0.1`
- [ ] Update runner manifest to v3.0.1, sync, restart pod
- [ ] Trigger a workflow_dispatch and verify `actions/checkout` succeeds

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/163

containers/forgejo-runner/Dockerfile

@@ -18,10 +18,17 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates \
     curl \
     git \
+    gnupg \
     jq \
     tzdata \
     && rm -rf /var/lib/apt/lists/*
 
+# Install Node.js (required by actions/checkout and other JavaScript Actions)
+RUN curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \
+    && apt-get install -y --no-install-recommends nodejs \
+    && rm -rf /var/lib/apt/lists/* \
+    && node --version
+
 # Install uv (Python package runner for towncrier)
 RUN curl -LsSf https://astral.sh/uv/install.sh | sh \
     && mv /root/.local/bin/uv /usr/local/bin/uv \