Ability for skill files to enable or declare additional MCP servers

### Describe the feature or problem you'd like to solve

Progressive disclosure: ability for Skill files to enable or declare additional MCP servers

### Proposed solution

In complex, autonomous, agentic workflows it is hard to anticipate which tools or sets of tools may be required along the way. Users are forced to enable all tools and toolsets (MCP servers) at the beginning of the session in case they may be needed by the agent during operation. This pollutes the context with unnecessary tools and degrades model performance.

Allowing a skill file to enable an MCP server (or register a new one) on skill activation would allow agents to only discover new tools when the skill is activated, based on its frontmatter (and thus when the tools are actually needed).

### Example prompts or workflows

_No response_

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by locating skill activation and MCP server configuration in the CLI, then trace how tools are discovered during a session. Done means a skill can enable or register an MCP server on activation, with its tools becoming available only when that skill is used.

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