docs(data-layer): add fetcher isolation rule

[pettinarip]

Adds a new rule to the data-layer skill (Rule 5) and a short pointer in src/data-layer/docs.md (Rule 4):

• Fetchers must stay isolated from the Next.js app — no process.cwd() reads, no imports from app/, src/components/, or src/lib/data/.
• Allowed: types, pure constants, pure utility functions.
• If a fetcher needs app data, fetch it via the GitHub API. Don't bundle app files via additionalFiles in trigger.config.ts.
• Points at fetchGitHubContributors.ts as the canonical pattern.

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[wackerow]

@pettinarip 👍 Lgtm, maybe just an explicit note about avoiding public/ files as well