feat: add optional working_dir to MCP and LSP toolset configs

[simonferquel-clanker]

Summary

MCP and LSP toolsets should support an optional working_dir field so they can be launched from a directory other than the agent's working directory.

Motivation

Some language servers (e.g. gopls) must be started from the Go module root. If the module lives in a subdirectory (e.g. ./backend), running the server from the repo root can cause resolution issues. Today there is no way to configure this.

Proposed config syntax

toolsets:
  - type: LSP
    command: gopls
    working_dir: ./backend

  - type: MCP
    command: my-mcp-server
    working_dir: ./tools/mcp

working_dir should be:

• Optional (defaults to the agent's working directory when omitted)
• Resolved relative to the agent's working directory if it is a relative path

Acceptance criteria

• working_dir field parsed from YAML/config for both MCP and LSP toolset types
• The toolset process is started with that directory as its CWD
• Relative paths are resolved relative to the agent's working directory
• Existing behaviour unchanged when working_dir is omitted
• Tests updated / added