Add givewell-donate: charitable donations via MCP

[ajfrai]

Adds givewell-donate, an MCP server for donating to GiveWell.

What it does: exposes donation as a tool call. Humans have card checkout flows; agents don't, so an agent holding funds has no ordinary way to give any of it away.

Notable design points:

• Three of five tools require no API key and no configuration (get_donation_address, prepare_donation, broadcast_signed_donation), so agents holding their own keys aren't gated on operator setup.
• Destination addresses are fixed in code — no parameter can redirect funds. The worst outcome, even from a compromised agent, is donating money it already had authority to spend.
• The relay path never receives a private key; the agent signs locally and the server only validates that the transaction pays GiveWell before broadcasting.
• Documents that agents should donate only funds they have authority to spend.

Supports BTC, ETH, and USDC. Install: npx -y givewell-donate

Verified working end-to-end against mainnet, including real donations.