docs: fix build warnings, PackageManagers component, Readme

[BlankParticle]

Clean website build warnings, render package-manager tabs from one component, and shorten preview install URLs.

<PackageManagers add={deps} dev={devDeps} />
<PackageManagers init dir="my-app" />
<PackageManagers run="alchemy deploy --stage prod" />

• PackageManagers replaces 171 hand-written tab blocks; tabs are now ordered pnpm, bun, npm, yarn. The CLI commands are now uniformly pnpm alchemy … / npx alchemy … (replaces npm run alchemy … and pnpm exec alchemy …).
• Install commands drop the Effect version specs (effect, @effect/* instead of @rc / @4.0.0-rc.115).
• Package-manager tab icons follow the new order and are scoped to pkgManager tabs (other tab groups no longer get the bun logo).
• Preview install URLs on main.alchemy.run use the short sha: pkg.alchemy.run/alchemy/a5ef336.
• starlight-blog override warnings: navigation: "none" (its header link lives in our empty ThemeSelect), and our MarkdownContent override is applied by a Starlight plugin that runs after starlight-blog.
• Starlight always reads an i18n collection; Astro 7 now warns when it's missing, so it gets one empty in-code en entry.
• gitignore code fence → txt (no Shiki grammar).
• README: working R2 Worker example (with imports), current provider list, beta (not alpha), and alchemy-run/alchemy@main (there is no v1 ref). Docs install alchemy@latest instead of alchemy@next.

[alchemy-version-bot]

Website Preview Deployed

URL: https://pr-2126-alchemy-website-preview.testing-2b2.workers.dev

Built from commit c6ae1c4.

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