feat: update naming convention docs

[LinKCoding]

Overview

Reworks Gamut's naming-convention documentation: the conventions themselves move into a new "Writing guidance" guide, and a new "Naming conventions rationale" concepts page explains the reasoning behind the less self-evident rules.

• Extracted naming conventions, code comments, file structure/naming, formatting, linking, and code-reference sections out of contributing-to-gamut.md into a new packages/gamut-docs/src/content/docs/guides/writing-guidance.md guide (authored first as .mdx, then renamed to plain .md since it has no MDX components).
• Rewrote contributing-to-gamut.md's component/story documentation section to reflect the Starlight site's actual workflow (Storybook autodocs instead of hand-authored .mdx docs, StoryEmbed/FigmaEmbed component-page conventions, sidebar registration notes).
• Added packages/gamut-docs/src/content/docs/concepts/naming-conventions-rationale.md, explaining the "why" behind naming rules (native HTML attribute reuse, enums over exclusive booleans, predominant-state defaults, logical vs. physical properties, interface-vs-implementation naming) with concrete examples from the Gamut component source.
• Registered the new guide in packages/gamut-docs/astro.config.mjs's sidebar and linked it from guides/index.md and concepts/index.md.
• Synced .claude/skills/gamut-writing/references/docs-in-code.md naming guidance to match the updated conventions (principles ordering, component props rules, RTL/logical-property rule).

PR Checklist

• Related to designs:
• Related to JIRA ticket: GMT-1801
• Changeset added (or empty changeset if no release)
• I have run this code to verify it works
• This PR includes unit tests for the code change
• This PR includes testing instructions tests for the code change
• Package previews from this PR are passing end-to-end tests in all relevant consuming repositories

Testing Instructions

Don't make me tap the sign.

1. Run yarn start:docs (or yarn start)
2. Visit /guides/writing-guidance/ — confirm the naming, comment, formatting, and linking sections render correctly and the page appears in the Guides sidebar.
3. Visit /concepts/naming-conventions-rationale/ — confirm it's listed on /concepts/ and each section's code examples (hideLabel, size="sm", leading/trailing, etc.) render with correct formatting.
4. (Optionally) Open .claude/skills/gamut-writing/references/docs-in-code.md and confirm its Naming section matches the same principles and ordering as the contributing guide.
5. (Optionally) Invoke the gamut-writing skill on a sample component edit and confirm it now recommends the new Component Props rules (e.g. native HTML attribute names, on-prefixed events).

PR Links and Envs

Repository	PR Link
Monolith	Monolith PR
Mono	Mono PR

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commit: 2676e40

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