fix: randomize heredoc delimiter in GITHUB_OUTPUT writes

[fg0x0]

Summary

Randomize the heredoc delimiter used when writing gemini_response and gemini_errors to $GITHUB_OUTPUT in action.yml.

The fixed EOF delimiter allows an LLM response containing a bare EOF line to close the heredoc early. Subsequent name=value lines in the response then become arbitrary step outputs, enabling bash injection in downstream consumer workflows.

Changes

• Replace echo "gemini_response<<EOF" with a random ghdelim_<hex> delimiter
• Replace echo "gemini_errors<<EOF" with a random ghdelim_<hex> delimiter
• Uses openssl rand -hex 16 per GitHub's canonical pattern

Testing

# Before fix: EOF in response breaks the heredoc
RESPONSE="line1\nEOF\ninjected=value"
echo "gemini_response<<EOF" >> output.txt
echo "$RESPONSE" >> output.txt
echo "EOF" >> output.txt
# Result: injected=value becomes a step output

# After fix: random delimiter is unguessable
_DELIM="ghdelim_$(openssl rand -hex 16)"
echo "gemini_response<<${_DELIM}" >> output.txt
echo "$RESPONSE" >> output.txt
echo "${_DELIM}" >> output.txt
# Result: entire response captured as gemini_response, no injection

Fixes #526

References

• GitHub docs: multiline strings
• Related: GHSA-62f2-6rx8-v262, PR fix: migrate shipped TOML templates from !{echo} to @{file} interpolation #524 (TOML template fix)

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[kkarrenn]

Thank you for this change!

[fg0x0]

@googlebot I signed it.