Fix multiline custom guidelines

[nathro]

Fix handling of multiline custom guidelines in the GitHub Action

• Use GitHub Actions multiline output syntax for $GITHUB_OUTPUT (context<<EOF ... EOF) so CUSTOM_GUIDELINES values with newlines are preserved intact.
• Keep the JSON shape produced by jq as { "custom_guidelines": <string> }, so downstream consumers remain unchanged.
• Maintain the existing fallback to {} when CUSTOM_GUIDELINES is unset.

Impact

• Prevents truncation and newline corruption when passing multiline guidelines between steps.
• Backwards-compatible; no breaking changes.

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[github-actions]

Reviewed the change to support multiline custom guidelines. Great move switching to the multi-line $GITHUB_OUTPUT block format — this will prevent issues with newlines.

I have one small, high-confidence improvement suggestion to make this even safer and more robust.

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[github-actions]

Nice switch to the multi-line output format. To further harden this and avoid any quoting/command-substitution pitfalls, consider:

1. Avoiding command substitution so you don’t lose trailing newlines and to keep very long outputs safe
2. Quoting $GITHUB_OUTPUT in case the path ever contains spaces
3. Using a slightly more unique delimiter than EOF

Example rewrite:

{
  {
    echo 'context<<GH_EOF'
    jq -n --arg guidelines "$CUSTOM_GUIDELINES" '{custom_guidelines: $guidelines}'
    echo 'GH_EOF'
  } >> "$GITHUB_OUTPUT"
}

This writes jq’s output directly without $(...), keeps formatting intact, and quotes the output file path.