feat: add agentic-cli-runner composite action (SC-18802)

[richard-webb-dev]

Summary

Adds a new reusable composite action agentic-cli-runner that runs OpenCode in non-interactive headless mode. This is a provider-agnostic action for AI-powered CI workflows, available to all repos in the simpleclub org.

Usage

- uses: simpleclub/.github/actions/agentic-cli-runner@main
  with:
    prompt: 'Your prompt here'
    agent_api_key: ${{ secrets.GEMINI_API_KEY }}
    agent_model: 'google/gemini-3-flash-preview'

This supersedes the PR in backend implementing it:
https://github.com/simpleclub/backend/pull/5534

I will close that one, and instead reference this repo instead.

Key Features

• Provider-agnostic — works with Google, Anthropic, OpenAI, OpenRouter, and 70+ others via provider/model string
• Single API key input — agent_api_key is applied to all common provider env vars; only the active provider uses it
• Retry logic — configurable max_attempts
• Metadata collection — processes .ai-metadata/*.json and *.md files created by the agent into a consolidated metadata output
• Artifacts — uploads metadata files, conversation history, and (on failure) zipped OpenCode logs
• CI-optimised — disables autoupdate, LSP download, and terminal title for lean execution

Testing

Tested locally with nektos/act using google/gemini-2.5-flash. OpenCode successfully answered a codebase question, created both expected metadata files, and the metadata was correctly extracted into the output.

example run:
https://github.com/simpleclub-extended/backend/actions/runs/22202068524

Files

actions/agentic-cli-runner/
  action.yml   — composite action definition (613 lines)
  README.md    — usage docs

[CLAassistant]

All committers have signed the CLA.

[richard-webb-dev]

Closing in favour of simpleclub/.github-private — action will be hosted there instead.