Add Extra Finance (xLend) lending plugin to base-mcp

## Feature request

Add a base-mcp skill plugin for **Extra Finance xLend**, the Aave V3-forked
lending market on Base. It would cover supply / withdraw / borrow / repay,
plus reads for single-reserve state and full account positions (health factor).

## Why

xLend is a live lending product on Base (USDC, WETH, wstETH, cbETH, cbBTC) and
is currently **not covered** by the base-mcp skill bundle. Agents can already
swap and bridge via base-mcp, but can't interact with this lending market.

## Proposed approach

Follows the official `send_calls` custom-plugin pattern:
1. A small viem-based runner (`npx extrafi-prepare`) reads state and encodes
unsigned calldata for the Pool / ERC20 contracts.
2. Emits `{ chain: "base", calls: [...] }`.
3. Agent passes it to Base MCP `send_calls`; user approves in Base Account.

I have a working implementation and would like to be assigned this issue so I
can open the PR. Happy to adjust scope to match project conventions.

Start by reviewing the official send_calls custom-plugin pattern and the npx extrafi-prepare runner described here, then compare its unsigned Pool/ERC20 calldata with existing base-mcp skills. Done means supporting supply, withdraw, borrow, repay, single-reserve reads, and full account positions with health factor, emitting Base calls for user approval.

Written by the indexing model from the issue text.