docs(clients): Add Multicoder extension - #1870

Add Multicoder extension to the clients list (VS Code section)

Multicoder VS Code extension allows switching between multiple agents, provides more visibility and better VS Code integration:

• 1-click install for 35+ agents from the ACP registry
• all your agent sessions in one place - CLI history included
• per-agent session params: model, thinking level, permission mode
• full session introspection: outline, reasoning blocks, tool calls, subagent runs
• changed files with aggregated and per-change diffs
• per-file change history - which session last touched it
• works in remote workspaces (SSH, WSL, containers)

Questions and ideas: GitHub Discussions

Bugs: GitHub Issues