Create the default Agents.md

### Current Behaviour and Context

There's currently no AI guidance in the repo

### Improved Behaviour

A basic agents file describing some basic actions such as how to start local server, some common gotchas, that this is a site on Altis and it's WordPress, composer for installing plugins etc

### Additional Info

This is a first pass, we'll see what else needs to be included based on the feedback that we get.

In the future when we update this, we want to let existing customers know.

### Acceptance criteria:
- [ ] Create an agents.md and have the claude.md read it
- [ ] Explain architecture
- [ ] Hot to start/stop stack
- [ ] How to run tests
- [ ] Common gotchas

## For Altis Team Use
### Ready for Work Checklist

Is this ticket ready to be worked on? See
[the Play Book Definition of Ready](https://playbook.hmn.md/play/product/definition-of-ready/)

- [ ] Is the title clear?
- [ ] Is the description clear and detailed enough?
- [ ] Are acceptance criteria listed?
- [ ] Have any dependencies been identified? (Optional)
- [ ] Have any documentation/playbook changes been identified? (Optional)
- [ ] Is an estimate or time box assigned?
- [ ] Is a priority label assigned?
- [ ] Is this ticket added to a milestone?
- [ ] Is this ticket added to an epic? (Optional)

### Completion Checklist

Is this ticket done? See
[the Play Book Definition of Done](https://playbook.hmn.md/play/product/definition-of-done-2/)

- [ ] Has the acceptance criteria been met?
- [ ] Is the documentation updated (including README)?
- [ ] Do any code/documentation changes meet project standards?
- [ ] Are automatic tests in place to verify the fix or new functionality?
- [ ] Or are manual tests documented (at least on this ticket)?
- [ ] Are any Playbook/Handbook pages updated?
- [ ] Has a new module release (patch/minor) been created/scheduled?
- [ ] Have the appropriate `backport` labels been added to the PR?
- [ ] Is there a roll-out (and roll-back) plan if required?

Start by locating the existing claude.md and the repository's architecture, stack commands, and test instructions. Create Agents.md covering the architecture, how to start and stop the stack, how to run tests, and common gotchas, then ensure claude.md reads it. Done means all listed acceptance criteria are checked.

Written by the indexing model from the issue text.