feat(skills): add Extra Finance xLend lending plugin reference

[mehmetkr-31]

Summary

Adds a base-mcp skill plugin reference for Extra Finance xLend, the Compound v3 / Aave V3-forked lending market on Base.

Problem

Extra Finance xLend is a live product on Base (supporting USDC, WETH, wstETH, cbETH, cbBTC) but was not covered in the base-mcp skill bundle. AI agents had no guidance on how to supply, withdraw, borrow, or repay on this market.

Solution

Create skills/base-mcp/plugins/extrafi-xlend.md describing:

• Read commands for active reserve rates and user positions via npx extrafi-prepare.
• Prepare write transaction batches (supply, withdraw, borrow, repay).
• Response mapping to send_calls for execution.
• Register the new plugin in skills/base-mcp/SKILL.md.

Fixes #63

[cb-heimdall]

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