Fix README updates in auto-release workflow

[Copilot]

The auto-release workflow attempted to append to a non-existent ## Release History section in README.md, causing silent failures. Additionally, version numbers in the README's Environment Variables table were never updated when new releases were detected.

Changes

• README.md: Added Release History section with table structure (Date | CUDA Tag | K3s Tag)
• Workflow: Consolidated README updates into single awk pass that:
  • Updates K3S_TAG and CUDA_TAG version numbers in Environment Variables table
  • Appends timestamped entries to Release History table

Implementation

awk -v e="$entry" \
    -v k3s="${{ steps.k3s.outputs.latest }}" \
    -v cuda="${{ steps.cuda.outputs.latest }}" '
  /^## Release History/ { print; getline; print; print e; next }
  /^\| `K3S_TAG`/ { sub(/`v[0-9][^`]*-amd64`/, "`" k3s "`"); print; next }
  /^\| `CUDA_TAG`/ { sub(/`[0-9][^`]*-base-ubuntu[0-9.]*`/, "`" cuda "`"); print; next }
  { print }
' README.md > README.md.new && mv README.md.new README.md

Replaces two separate steps (sed for version updates, awk for history) with unified awk command that preserves formatting.

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