Add `AGENTS.md` to templates

[sed-i]

What needs to get done

Come up with a set of instructions for agentic coding.

Refs:

• https://agents.md/
• https://claudecode.io/tutorials/claude-md-setup
• AGENTS.md files on github - query

Why it needs to get done

To enable more productive agentic coding.

[lengau]

I'm not sure how useful this file would be inside the templates, since people would have to fill in most of the information themselves. Could you suggest something that would be decently useful but still generic enough that it would apply to all agents?

[tonyandrewmeyer]

I think it's reasonable to have things that work across agents, although picking a couple (until they all standardise more) might work well. I did some playing around with this a couple of months ago, and have done a little more since but haven't had time to write it up.

I think "how do we make agentic LLM development in charming better" would be a great discussion for the sprint if you'd both be up for that?

[sed-i]

Statements I thought of, myself

## Intent
- This charm is an opinionated operator that prioritizes stability and upgradeability.
- The charm's interface to admins is its config options and actions.
- The charm's interface to other charms is via juju relation data, declared under the
  "provides" and "requires" section of charmcraft.yaml.

## Constraints
- Downloaded artifacts should not be a runtime dependency (part of charm's python code) 
  but a pack-time dependency, as a dump plugin in charmcraft.yaml.
- The python features used should be available on the lowest version of python per
  "platforms" section in charmcraft.yaml.
- Operations should be idempotent, i.e. running the same hook multiple times should provide
  the same result.

Auto-generated by Cursor

I asked Cursor to generate one for me and the result so good I'm considering using it for README.md for that charm!

AGENTS.md

I imagine we could provide a general AGENTS.md in the template and charm devs could then (auto) update it.

Here's the prompt:

Based on the context of this project, as well as 
@https://canonical-charmcraft.readthedocs-hosted.com/en/stable/reference/files/charmcraft-yaml-file and
@https://canonical-craft-parts.readthedocs-hosted.com/latest/reference/parts_steps/
create an AGENTS.md file

Update the AGENTS.md file with relevant information from 
@https://documentation.ubuntu.com/ops/latest/tutorial/write-your-first-machine-charm/