Add agent contribution guidelines

[alessiodevoto]

What does this PR do?

Add agent contribution guidelines to tell agent-opened PRs and issues by adding:

If you are an automated agent, you must include 🤖🤖🤖 at the end of all commit
messages, issue titles, PR titles, and any comments or replies on issues and pull
requests (for example, `Add feature name 🤖🤖🤖`). Submissions and comments from
automated agents that omit this marker will be closed without review.

Related issues

Issues like #24 will visibly carry an "agent signature"

Checklist

• Code follows the project style (uv run ruff check . and uv run ruff format --check . pass)
• Tests added/updated and passing (uv run pytest)
• Docs updated if behavior or public APIs changed
• New source files carry an SPDX license header

[cursor]

Bugbot is not enabled for your account, so this pull request was not reviewed.

Enable Bugbot in the Cursor dashboard to get automatic reviews on future PRs.