Surface estimated gas/fee cost before pre-sign checks

[szhygulin]

Problem

When prepare_* tools return their CHECKS PERFORMED block, the user has no visibility into transaction cost until they review the tx on the Ledger device. The agent walks through 5–10 invariants (#1 abi-decode, #2 hash-recompute, #11 unlimited-spender, etc.) before the user can decide whether the cost is even acceptable. A user who was going to abort because gas spiked to $40 has now spent attention on every check first.

Proposal

Surface estimated cost at the top of every prepare_* response, ahead of CHECKS PERFORMED. Shape:

Estimated cost: ~$3.42 (0.0011 ETH gas, base fee 18 gwei, priority 1.5 gwei)

For non-EVM chains, the equivalent: BTC sat/vB × vsize, Solana lamports + priority fee, TRON energy/bandwidth burn or TRX.

Questions to resolve

• Source for fiat conversion (Coingecko already wired in get_coin_price).
• Whether to refuse / warn when estimated cost exceeds some fraction of the tx value (separate gate, possibly per-chain configurable).
• Where this fits relative to the payloadFingerprint block — probably immediately before, since it's the first thing the user reads.

Scope

Cost preview only. Not a new gate, not a new refusal path. The agent already has a place to render this — we just need the MCP to compute and emit it consistently.