Add fxusd skill for fxSAVE shortcut flow

[huwangtao123]

Summary

• add a new fxusd skill for minting and redeeming Base fxSAVE
• document the public app backend at https://fxsave.up.railway.app
• include API reference and a small CLI helper for bundle / approval planning
• add the skill to the root README list

What the skill does

The skill shortcuts the manual cross-chain flow for fxSAVE on Base.

Instead of manually thinking through:

• bridge from Base to Ethereum mainnet
• deposit or redeem on mainnet
• bridge the result back to Base

it lets the user or agent work at the shortcut level:

• mint Base fxSAVE from fxUSD, USDC, or WETH
• redeem Base fxSAVE back into Base assets

Example use cases

• Deposit 10 fxUSD to fxSAVE
• Deposit 100 USDC to fxSAVE
• Redeem 50% of my fxSAVE to fxUSD
• Preview the route to mint fxSAVE from 1 WETH

Validation

I tested the public Railway backend directly with the bundled CLI helper:

• python3 fxusd/scripts/fxusd_cli.py mint --from-address 0x241e25c9d15b5E7FB007B5E028C4ce8694893870 --amount 1 --source-token fxUSD
• python3 fxusd/scripts/fxusd_cli.py redeem --from-address 0x241e25c9d15b5E7FB007B5E028C4ce8694893870 --amount 1 --target-token USDC

Both returned:

• 200 bundle responses
• 200 approval responses
• valid executable transaction payloads from the public app backend

[huwangtao123]

Adding a bit more context here:

This skill is intended to be the starting point for a broader collection of fxUSD-related shortcuts, not just a single mint/redeem wrapper.

The initial version focuses on the most obvious Base user flow:

• mint fxSAVE from Base assets
• redeem fxSAVE back into Base assets

But the direction is larger than that.

The goal is to make fxUSD yield and arbitrage workflows easier to access from Base by packaging the hidden cross-chain steps behind simpler agent-friendly actions. Over time, this can grow into a set of shortcuts around:

• stable yield routing
• fxUSD -> fxSAVE entry flows
• fxSAVE -> Base asset exit flows
• basis / arbitrage / rebalancing style flows built on top of the same pattern

In short: this PR adds the first public shortcut skill, but the broader vision is an fxUSD shortcut collection for yield and arbitrage-related actions.