skill tool alias for custom agent profiles

[alireza-newday]

Describe the feature or problem you'd like to solve

Custom agents ( .github/agents/*.agent.md ) support a  tools:  frontmatter list, but the documented aliases are limited to  execute ,  read ,  edit ,  search ,  agent ,  web ,  todo. there's no way to grant a custom agent access to the  skill  tool that interactive sessions have. This forces duplicating an installed skill's full instructions (format, rules, examples) verbatim into the agent's markdown body instead of referencing it live, so the two drift out of sync whenever the skill is updated.

Proposed solution

1. Add  skill  as a recognized tool alias in the  tools:  frontmatter list (alongside  execute ,  read ,  edit ,  search ,  agent ,  web ,  todo ), mapping to the same  skill  tool exposed in interactive sessions.
2. When declared, the custom agent's runtime tool set includes  skill(name: string) , resolved against the same skill-discovery order already used interactively (project → user → installed plugins), so no new skill-registration mechanism is needed.
3. Optionally, allow scoping to specific skills via  tools: ["skill/ponytail-review"]  (consistent with the existing  some-mcp-server/some-tool  namespacing pattern for MCP tools), so an agent can be restricted to only the skills it needs rather than all installed skills.
4. No change needed to skill file format ( SKILL.md  + frontmatter). this is purely a tool-provisioning change on the custom-agent side.

Example prompts or workflows

A PR-review custom agent ( dotnet-pr-reviewer.agent.md ) that wants to apply the  ponytail-review  skill's methodology by invoking  skill(name: "ponytail-review")  directly at review time, instead of embedding a static copy of  ponytail-review/SKILL.md  in its prompt that must be manually kept in sync.

Additional context

No response