chore: simplify contributing workflow and documentation

[drew]

Summary

Contributing flow is harder than it needs to be because task entrypoints and contributor docs are too broad.

Goals

• Keep a small, clear set of public mise tasks for day-to-day development.
• Simplify CONTRIBUTING.md for fast onboarding.
• Move style conventions into STYLE_GUIDE.md.
• Document the local ncl shortcut and the main tasks contributors should use.

Proposed Work

• Restructure task visibility so only the primary developer commands are public.
• Rewrite CONTRIBUTING.md to focus on prerequisites, quick start, project structure, and main tasks.
• Keep commit message and pull request guidance in CONTRIBUTING.md.
• Place DCO guidance under pull request expectations.

Acceptance Criteria

• mise tasks ls shows only the primary contributor task set.
• CONTRIBUTING.md is concise and task-focused.
• STYLE_GUIDE.md exists and contains code/CLI style guidance.
• Docs reference cluster:sandbox, test:e2e, and version consistently.