GitHub Action: coreball-action for PR context comments

[francomano]

Problem

Teams want CoreBall context packs in CI — e.g. post relevant files as a PR comment or feed an LLM review. No reusable Action exists.

Proposal

Create coreball-action (composite or Docker) that:

• runs coreball pack --task \"$PR_TITLE $PR_BODY\" --max-tokens 2048 --format markdown
• posts the package as a PR comment (or artifact) with actions/github-script
• optional: sends pack to an LLM endpoint and comments the summary
• published to GitHub Marketplace (action.yml + branding)

Benefits

• Marketplace discovery is a top star driver — every uses: francomano/coreball-action@v1 is a backlink.
• Unlocks “AI Code Review” workflows — trending topic in 2026.
• Reuses existing coreball pack + HTTP API — no new core code.

Acceptance criteria

• action.yml with inputs: task, max_tokens, repository, comment (bool)
• E2E tested on a sample repo (comment appears on PR)
• Published to Marketplace (docs + coreball-action repo or action branch)
• README section + docs/marketplace.md

Good first issue — mostly YAML + docs.