Support Copilot plugins in ExperimentConfiguration

[Copilot]

Experiments could configure MCP servers, instructions, skills, and custom agents, but not Copilot plugins. This adds declarative plugin loading mirroring the existing mcp.servers setup.

Changes

• config.yaml: new plugins section with marketplaces (extra registries) and install (list of <plugin>@<marketplace>).
• shared/plugins.py: install_plugins() registers marketplaces, installs plugins via the Copilot CLI, and returns the installed names.
• ExperimentConfiguration: added plugins: list[str] | None; included in is_empty().
• Copilot agent: installs plugins before running and records them; CLI lookup moved earlier to pass copilot_cmd. Copilot-only (other agents ignore the section).
• Display / EXPERIMENT.md: surface plugins alongside MCP servers; tests for parsing and install.

Usage

plugins:
  marketplaces: []            # extra OWNER/REPO or local paths
  install:
    - "database-data-management@awesome-copilot"