docs: starter framing, signup path, and credit terms

[kuanzema]

What changed

• Top-level README reframed from event-scoped ("Demo at Harvard") to the evergreen HarnessRouter Starter; the Harvard workshop provenance now lives only inside the two demo READMEs, per positioning guidance.
• New Get your API key section closing the acquisition loop: free signup link, the card requirement to unlock the 500-credit launch offer, and the clarification that demos install and start without a card — credits are only consumed when tasks run through the API key.
• Added website / docs / pricing / example-video links with ?ref=github-starter attribution.
• LumaCare README notes its Harvard workshop origin (the Cursor demo README already did).

Repo settings updated alongside this PR: renamed to harnessrouter-starter (old URLs redirect), homepage set to harnessrouter.ai.

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