Add capability declaration fields to SKILL.md frontmatter (tools, mcp-servers, hooks, model)

## Brief

**Canonical issue:** https://github.com/microsoft/vscode-copilot-chat/issues/5101

Cross-filed at the GitHub Copilot CLI repo because SKILL.md is a cross-tool format consumed by both VS Code Copilot Chat and Copilot CLI plugin install. The gap and the proposal apply identically here.

## Summary (1 paragraph)

Custom agents (`.agent.md`) declare their capabilities in frontmatter — `tools`, `mcp-servers`, `hooks`, `model`. Skills (`SKILL.md`) cannot — they currently support only `name`, `description`, `argument-hint`, `user-invocable`, `disable-model-invocation`. The asymmetry forces every consuming agent to know and re-declare a skill's capability requirements, leading to silent under-capability and ecosystem-specific workarounds. Proposed: extend SKILL.md frontmatter with the same optional fields, with skill-declared `tools`/`mcp-servers` unioning with consuming agents, skill-declared `hooks` activating with the skill, and `model` as advisory.

## Why filed here

`copilot plugin install` loads SKILL.md files; the same capability-declaration gap applies to the CLI surface. Closing it natively at the SKILL.md level reduces the need for plugin-level bundling workarounds (whole plugin enables together, coarser than per-skill) and aligns skill composability across both surfaces.

See the canonical issue for full proposal, use cases, inheritance semantics, and references.

Start with the canonical issue at microsoft/vscode-copilot-chat#5101 and trace where copilot plugin install loads SKILL.md files. Determine the relevant frontmatter parsing and capability-inheritance entry points, then validate that the proposed fields and semantics work across the CLI and VS Code surfaces with appropriate tests.

Written by the indexing model from the issue text.