Add agent instructions and writing rules

[xylar]

This PR adds AGENTS.md, a CLAUDE.md that points at it, and five writing skills under .claude/skills/, so that AI agents working in this repo follow Compass conventions instead of inventing their own.

AGENTS.md covers the parts of the repo an agent gets wrong without being told: that pixi-env/ is the development environment and that creating it is a developer action, that deploy.py and deploy/cli_spec.json are contracts shared with mache and must be changed there first, that documentation is reStructuredText needing pages in both guides plus an api.rst entry, and that the machines table in the Developer's Guide has to stay consistent with compass/machines/.

The writing rules are the other half. Agent-written GitHub prose has been running an order of magnitude longer than what colleagues write, and length is the whole complaint. Each skill carries the rules for one artifact, calibration measured from Compass's own 2023 and 2024 history, and worked examples quoted from real pull requests. The Too much examples are constructed in the style of real agent-written prose from elsewhere, and are labeled as such.

These files are adapted from the equivalent ones in Polaris. The environment, contract and human-writing sections carry over. The style, documentation and supported-machine sections follow Compass's own tooling, and every calibration number and example was re-measured against MPAS-Dev/compass.

Checklist

• Document (in a comment titled Testing in this PR) any testing that was used to verify the changes

🤖 Generated with Claude Code

[xylar]

Testing

I ran pre-commit on the seven added files via pixi exec pre-commit, since no pixi-env/ exists in this worktree. trailing-whitespace and end-of-file-fixer passed; isort and flake8 were skipped because nothing here is Python.

The calibration numbers in each skill were measured from the GitHub API over MPAS-Dev/compass pull requests, reviews and comments created in 2023 and 2024, excluding bots. The design-document numbers come from the two files in docs/design_docs/. Every quoted example is linked to the PR it came from.

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Posted by Claude Code on @xylar's behalf. The testing, analysis and wording above are AI-authored; please check them accordingly.

[xylar]

@matthewhoffman and @trhille, an attempt to keep Claude and Codex from spewing walls of text.