docs: apply the Variable Lab README structure, and add the contributing and development guides

[rqbazan]

Applies the README structure from the vlab-brand skill's readme.md (brand#15).

What's in it

• README.md (a Project in its own repo):
  • the Sign with a width and no height, so GitHub keeps its corners square;
  • the name in the h1 links to https://lab.variable.land/theme-toggle;
  • a new opening that doesn't repeat the tagline, and ## Install with the requirement from the React peer;
  • ## Development moves to docs/development.md, replaced by ## Contributing;
  • the License line and the Signature.
• Upgrading title: it becomes "Upgrading from 1.x", because no title holds a scoped package name. Its first sentence names both packages. No repo linked the old anchor.
• LICENSE: the holder is now "rcrd ricardo@variable.land".
• docs/contributing.md and docs/development.md: checked against mise.toml, package.json, the workflows and lefthook.yml.
• Changeset: a patch for @variablelab/theme-toggle, because the README and the LICENSE ship in the tarball.

Verified

• pnpm rr check and pnpm test pass (25 tests).
• npm pack --dry-run lists LICENSE and README.md.
• On the branch, in light and dark: square corners, the h1 link, the Signature aligned, and no table splits a name. GitHub shows the Contributing tab.
• The README's links to blob/main/docs/… resolve after the merge.

🤖 Generated with Claude Code

https://claude.ai/code/session_01XRCpaD5QTPJsEjnhikoV2S

[changeset-bot]

🦋 Changeset detected

Latest commit: 442e1ad

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@variablelab/theme-toggle	Patch

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