fix: replace vercel-labs/zero slug with vercel-labs/zerolang across docs and installer

[HariNayan]

fix: replace vercel-labs/zero slug with vercel-labs/zerolang across docs and installer

Closes #167

Problem

Several user-facing files still defaulted to the old vercel-labs/zero
repository slug. GitHub redirects it today but the name is wrong and
could silently break if the redirect is ever removed — affecting the
installer download, star count API calls, header links, agent skill
commands, and the docs-chat system prompt all at once.

Changes

Six string replacements across the docs layer, no logic touched:

File	What changed
docs/public/install.sh	ZERO_REPO default: vercel-labs/zero → vercel-labs/zerolang
docs/lib/github.ts	REPO constant fallback for GitHub star count API
docs/components/site-header.tsx	GitHub header href fallback
docs/components/install-copy.tsx	npx skills add command string shown to users
docs/articles/install.md	Install article prose reference
docs/app/api/docs-chat/route.ts	SYSTEM_PROMPT GitHub repository URL

Verification

Search the repo for vercel-labs/zero" (with closing quote) —
no results should remain. All six affected lines now read
vercel-labs/zerolang.

No tests need updating. No command contracts change.
No compiler behavior is affected.

[vercel]

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