feat(cli): support GitHub registries

[maxffarrell]

Closes #2725.

Depends on #2722, which is now merged.

Summary

• port GitHub source registries from shadcn/ui, including item addresses, source includes, ref resolution, source-aware dependency deduplication, structured validation, docs, and the June changelog
• port private-repository support with anonymous-first reads and GH_TOKEN, GITHUB_TOKEN, or stored gh credentials
• pin authenticated reads to commit SHAs, isolate credentials to api.github.com or gh, cap source files at 5 MiB, and preserve anonymous behavior for public sources
• port scoped registry context, awaited and deduplicated auth notices, sanitized diagnostics, bounded validation and gh concurrency, and real subprocess coverage
• port the August private GitHub registries changelog and add a minor CLI changeset

Verification

• pnpm lint
• pnpm --filter shadcn-svelte check
• pnpm --filter shadcn-svelte test -- --run (30 files, 327 tests)
• pnpm --filter docs check (0 errors, 0 warnings)
• pnpm --filter docs build
• git diff --check

Upstream references: shadcn/ui commits 7c63c467, cdd7d554, 33c81f99, e68a185a, and f0ae33a8.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0d7ada6

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

This PR includes changesets to release 2 packages

Name	Type
shadcn-svelte	Minor
@shadcn-svelte/parity	Patch

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