Docs request: Tutorial on extending skills with new customization dimensions

## What I'm asking for

A tutorial or guide that walks through how to add a **new customization axis** to an existing gstack skill — not just locale, but any dimension: verbosity level, user role, project type, tone, output format, etc.

## Background

Right now gstack skills are mostly fixed-behavior prompts. Issue #481 shows that people are already forking skills to add locale awareness, but there's no shared playbook for *how* to approach this kind of extension cleanly.

The questions I keep running into:

- Where should the customization value live? (`gstack-config`, env var, per-project config, frontmatter?)
- How should a skill read it without breaking when it's missing?
- What's the pattern for keeping the default behavior intact while allowing overrides?
- Is there a recommended way to document new config keys so users know what's available?

## Why this matters

Once contributors know the approved pattern, multiple customizations (locale, verbosity, persona, etc.) can coexist without stepping on each other. Without a pattern, everyone solves it differently and skills become hard to maintain.

## What a good tutorial would cover

1. Pick one concrete example (locale is fine) and walk through it end-to-end
2. Show where config values are stored and how to read them defensively in a skill
3. Show how to keep `English / default behavior` as the fallback
4. Note any gotchas (e.g. zsh vs bash compatibility, reading config in the preamble block)

Happy to help draft or review if there's interest.