build: add local Go workspace

[CharlieLZ]

Summary

• Add a committed go.work so local Go commands use the vendored keep-common module already present in this repository.
• Route github.com/btcsuite/btcd to the vendored ./btcd tree for normal local checkouts instead of requiring the Docker-only ../btcd sibling path.
• Include go.work.sum generated by the verified workspace run.

Why

A standard clone currently cannot run even focused Go tests because the root go.mod replacement expects ../btcd, a path created by the Dockerfile but not by a normal checkout. The repository already contains ./btcd and ./keep-common; this workspace makes those local modules usable without changing production code.

RED before this change:

go test ./config
# github.com/keep-network/keep-core/config
pkg/bitcoin/block.go:7:2: github.com/btcsuite/btcd@v0.23.1: replacement directory ../btcd does not exist
config/config.go:21:2: missing go.sum entry for module providing package github.com/keep-network/keep-common/pkg/chain/ethereum

Verification

• go test ./config now passes.
• go test ./cmd ./config passes.
• git diff --cached --check passes before commit.

Bounty context

Submitted for Bitgesell improvement bounty: BitgesellOfficial/bitgesell#81

Payment details can be provided if accepted or requested.