feat: compile the output style from writing references

## Problem

`references/output-style.md` is maintained by hand. Claude Code installs the file as a standalone prompt that cannot load the skill's linked references. Compact rules such as cute labels can drift from `writing-style.md` and the detailed examples.

## Proposed change

Generate the output style from an explicitly selected subset of the writing rules. Keep the detailed examples in the skill references so the standalone prompt remains small.

## Acceptance criteria

- One source defines each rule shared by the editor and output style.
- A deterministic command regenerates `references/output-style.md`.
- Validation fails when the generated file is stale.
- The generated output style remains concise and standalone.
- The README explains that output-style changes apply after `/clear` or in a new session.

Start by reading references/output-style.md and references/writing-style.md, then inspect the repository's existing build or validation entry points. Define the shared rules, add a deterministic regeneration path and stale-output validation, and update the README so the output style remains concise, standalone, and its session-refresh behavior is documented.

Written by the indexing model from the issue text.