feat: add minimal EIP-1193 browser bridge scaffold

[0xpantera]

Summary

• Move the EIP-1193 browser bridge out of deckard-mcp into a standalone deckard-browser-bridge crate.
• Add deckard-wallet-client for shared signer client access, chain id config, wallet address lookup, and common failure mapping.
• Keep MCP focused on the MCP/agent interface while the browser bridge owns loopback HTTP, EIP-1193 request handling, in-memory sessions, and dev/mock account mode.
• Update docs and repo map to describe MCP and browser bridge as sibling interfaces over shared wallet capabilities.

Why

MCP is an agent interface; the browser bridge is a dapp/browser interface. The spike lived under deckard-mcp because it reused the existing local sidecar/API surface, but keeping HTTP/EIP-1193/browser-extension logic there would make MCP a generic local API server. This refactor gives the browser bridge its own boundary without expanding its API.

How to run

Browser bridge:

cargo run -p deckard-browser-bridge -- --bind 127.0.0.1:8765

Dev/mock mode:

cargo run -p deckard-browser-bridge -- \
  --bind 127.0.0.1:8765 \
  --dev-mock-account 0xdeC0ded0000000000000000000000000000001193

Test dapp:

python3 -m http.server 8777 --directory examples/browser-bridge-dapp

Intentionally experimental / unchanged

• Extension remains keyless.
• Sessions remain local/dev/in-memory.
• No signing methods added.
• No Kohaku, hardware wallet, or native messaging expansion.
• Unsupported methods still fail cleanly with EIP-1193-style 4200 errors.

Follow-up cleanup ideas

• Harden browser transport/origin model.
• Add explicit approval UI.
• Add persisted permission registry.
• Add EIP-6963 provider discovery.
• Add an integration test for the /rpc HTTP boundary.

Verification

cargo fmt --all --check
just check
cargo test --workspace

All passed locally.