Define a common `CLAUDE.md` structure for teal repositories

## Description

We would like to introduce `CLAUDE.md` files across the teal repositories to provide useful context and guidance when working with Claude Code.

Claude Code provides `/init` to analyze a repository and generate an initial `CLAUDE.md`. Before adding these files to individual repositories, we should first agree as a team on what kind of information we want them to contain and whether there are common categories or conventions that should be consistent across teal repositories.

The goal of this issue is to explore and define a common starting point for `CLAUDE.md`, rather than creating the files for every repository yet.

Repositories in scope include:

* `teal`
* `teal.data`
* `teal.code`
* `teal.reporter`
* `teal.logger`
* `teal.widgets`
* `teal.picks`
* `teal.slice`

## What to consider

As part of this issue, explore what makes a useful `CLAUDE.md` for teal repositories. Claude Code's `/init` output can be used as a starting point, together with any relevant recommendations or examples.

Consider what categories of information would be useful for Claude when working in a teal repository, what should be common across the ecosystem, and what should remain repository-specific.

The outcome should be a lightweight proposed structure or template that can be discussed with the team and used as the starting point for introducing `CLAUDE.md` files to the individual repositories.

## Acceptance criteria

* A proposed `CLAUDE.md` structure has been reviewed by the team.
* The expected categories/content are agreed upon.
* There is a reusable starting point for follow-up work to introduce `CLAUDE.md` to the teal repositories.

### Related Task:
* https://github.com/insightsengineering/teal.code/issues/290
* https://github.com/insightsengineering/teal.widgets/issues/360
* https://github.com/insightsengineering/teal.picks/issues/122
* https://github.com/insightsengineering/teal.logger/issues/126
* https://github.com/insightsengineering/teal.data/issues/391
* https://github.com/insightsengineering/teal.slice/issues/691
* https://github.com/insightsengineering/teal/issues/1728
* https://github.com/insightsengineering/teal.reporter/issues/466
* https://github.com/insightsengineering/teal.modules.general/issues/1017
* https://github.com/insightsengineering/teal.modules.clinical/issues/1548
* https://github.com/insightsengineering/teal.osprey/issues/358
* https://github.com/insightsengineering/teal.goshawk/issues/390