📖 docs: explain the contributor relay in the README intro

[clubanderson]

Adds a "Contributor relay: a swarm working on your behalf" section after Hub and spoke, mirroring hivecommons/docs#348: contributors donate compute and model tokens by running their own agent; their API keys never reach the hive; no collaborator grants — the hive mints a short-lived task-scoped GitHub token per task; same prompts/limits/gates. Cross-links the existing "Contribute to a Hive" section.

Closes #12122

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[github-actions]

Open PR overlap check

Base branch: v5

Conflicts are checked with every PR merged onto the current tip of v5, so a PR that only needs a rebase is not reported as conflicting with its siblings.

No conflicting open PRs or same-file overlaps were found.

[hivecommons-hive]

merged by Hive trusted-author auto-merge: author @clubanderson holds role owner and GitHub permission admin on repo; CI green at 1b15585.

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