docs: Add AGENTS.md to guide AI coding assistants

[planetf1]

Misc PR

Type of PR

• Bug Fix
• New Feature
• Documentation
• Other

Description

• Link to Issue: Add AGENTS.md to guide AI coding assistants #319

Adds structured guidance for AI coding assistants (Claude Code, Cursor, Copilot, etc.) working with Mellea.

Closes #319

What's included

• AGENTS.md - Guidelines for contributors modifying Mellea internals (test markers, coding standards, common issues)
• docs/AGENTS_TEMPLATE.md - Template that downstream projects can copy when building apps with Mellea

Why

AI assistants without guidance could produce code that doesn't follow project patterns. This follows the emerging AGENTS.md convention (https://docs.anthropic.com/en/docs/claude-code/memory#agentsmd).

Feedback welcome on what to add, remove, or clarify. I missed the statement about getting comments on issue first - apologies. I'm sure the dev team will have feedback on the rules, which I built from best practice/previous projects & what I've experienced with mellea over a few days using it. I do think it's important we get something to guide towards consistency?

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code as added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

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[nrfulton]

I missed the statement about getting comments on issue first - apologies.

No worries!

I'm sure the dev team will have feedback on the rules, which I built from best practice/previous projects & what I've experienced with mellea over a few days using it. I do think it's important we get something to guide towards consistency?

I haven't thought deeply about a generic Mellea AGENTS.md; I normally write a separate one for each major project I'm working on. But having a generic AGENTS definitely makes sense.

I'll bring this up in our leads today. I think there are two paths:

1. Do it right the first time. That probably creates some iteration on this PR.
2. Just add something to get started and let the community iterate. Then we'd probably just merge this PR soon with the expectation of future iteration.