docs: refine Copilot generalization design

[nighca]

Refs #2808

Summary

• add the Copilot generalization design doc under docs/develop/generalization/copilot.md
• define an in-memory skill registry model built from fs-like skill bundles
• document tool-driven skill activation, markdown resource loading, and structured tool-result wrapping
• clarify module boundaries between CopilotRoot, the generic Copilot core, project-specific integrations, and the separate skill-support module
• note related follow-up work in Make Copilot CodeBlock and CodeChange project-type-agnostic #3043, Auto-activate editor Copilot skills by project type #3045, and Bundle class-framework Copilot skills from installed packages at build time #3046

Notes

• documentation-only change
• no tests were run

[gemini-code-assist]

Code Review

This pull request outlines the technical design for generalizing XBuilder's Copilot, moving from a project-specific implementation to a modular, skill-based architecture. Key changes include decoupling platform-level logic from project-specific context, introducing an in-memory skill registry, and implementing a progressive disclosure model via a new load_skill tool. Review feedback identifies inconsistencies in the proposed TypeScript interfaces and contradictions between the tool implementation snippets and the specified output format.

[gemini-code-assist]

The LoadedSkillDocument type includes resourcePaths: string[]. However, the examples in Section 6 show that only the main skill document lists resources, while individual resource documents do not. To align with this, consider making resourcePaths optional so it can be omitted for resource documents.

[gemini-code-assist]

This code snippet shows the tool returning a LoadedSkillDocument object directly. This contradicts Section 6, which specifies that the tool should return a structured XML-like string (e.g., wrapped in <skill_content>). The implementation should be updated to include the formatting step to ensure consistency within the design.