Implement `replace` tool to replace a message in the mempool

[sudo-shashank]

Summary

Lotus provides lotus mpool replace to re-broadcast a pending message with higher gas (replace-by-fee), so messages stuck in the mempool (low GasFeeCap / GasPremium vs current base fee) can get mined.

Forest supports RBF in the message pool internally but has no CLI to replace a pending message. Users hitting stuck sends must work around it manually.

Background / Lotus reference
Command: lotus mpool replace

NAME:
   lotus mpool replace - replace a message in the mempool

USAGE:
   lotus mpool replace [command options] <from> <nonce> | <message-cid>

OPTIONS:
   --gas-feecap value   gas feecap for new message (burn and pay to miner, attoFIL/GasUnit)
   --gas-premium value  gas price for new message (pay to miner, attoFIL/GasUnit)
   --gas-limit value    gas limit for new message (GasUnit) (default: 0)
   --auto               automatically reprice the specified message (default: false)
   --fee-limit max-fee  Spend up to X FIL for this message in units of FIL. Previously when flag was max-fee units were in attoFIL. Applicable for auto mode
   --help, -h           show help

Completion Criteria

• Replace by updates gas and pushes replacement; old pending message superseded via RBF also works using .
• --auto reprices using RBF minimum + gas estimation; respects --fee-limit/max-fee when set
• Integration test

Additional Links & Resources