[Docs]: CopilotSkill global mode limitation and .instructions.md naming rule are undocumented

[flanny7]

Hello, I always appreciate rulesync.

Problem

Two Copilot-specific behaviors exist in the implementation but are not mentioned in any documentation.

Issue 1: CopilotSkill throws on global mode

In src/features/skills/copilot-skill.ts around line 74–76:

throw new Error("CopilotSkill does not support global mode.");

Users who attempt rulesync generate --targets copilot --features skills in global mode will receive an error with no prior warning or documentation explaining this constraint.

Issue 2: Non-root rule files use .instructions.md double extension

Non-root Copilot rules are generated as .github/instructions/<name>.instructions.md. The double .instructions.md extension is a GitHub Copilot convention, but this is not explained anywhere in the docs. Users unfamiliar with this convention may be confused by the generated filenames.

Proposed Solution

Both limitations should be documented in the dedicated Copilot page (once created) or in docs/reference/supported-tools.md:

1. Add a note that skills feature is not available in global mode for copilot
2. Explain the .instructions.md naming convention and its origin

Your consideration would be appreciated.