feat: optional validation — Co-Authored-By trailer, if present, must specify a human, not an AI agent

[javier-godoy]

Summary

As AI-assisted development becomes more common, commit messages may include Co-Authored-By: trailers that credit AI agents (GitHub Copilot, Claude, etc.). The Co-Authored-By trailer is a Git convention for crediting human co-authors, and crediting an AI agent with this trailer misrepresents authorship.

This issue requests an optional validation rule that fails a commit if any Co-Authored-By trailer names a known AI agent instead of a human.

Proposed Behavior

When the feature is enabled, the action should:

1. Parse the commit body for Git trailers — lines of the form Token: value appearing after a blank line.
2. For each Co-Authored-By: Name <email> trailer found, check whether Name or email matches a known AI agent pattern.
3. If a match is found, fail the commit with a message such as:

   Co-Authored-By trailer must specify a human author, not an AI agent

The check must be opt-in (disabled by default), enabled via an action input, for example:

- uses: FlowingCode/action-conventional-commits@vX.Y.Z
  with:
    disallow-ai-coauthors: true

Known AI Agent Patterns (initial list)

The detection should cover at least the following well-known patterns (name or email):

Agent	Typical trailer value
Anthropic Claude	... <noreply@anthropic.com>
GitHub Copilot	GitHub Copilot <copilot@github.com>
GitHub Actions bot	... <...github-actions[bot]...>
Generic bot accounts	name or email containing [bot]

The list should be extensible (e.g., a hardcoded array in source that can grow over time).

Acceptance Criteria

• The feature is disabled by default; existing users see no behavior change.
• When enabled, commits with a Co-Authored-By trailer matching a known AI agent pattern are rejected with a clear error message.
• Commits with no Co-Authored-By trailer, or with a trailer that does not match any AI pattern, pass normally.
• Unit tests cover: no trailer, human trailer, known AI trailer (each known agent), and unknown email (should pass).
• action.yml is updated with a documented disallow-ai-coauthors boolean input (default false).
• README documents the new input and the validation rule.