Add explain-issue skill

[dyoshikawa]

Summary

• add a new explain-issue official skill under .rulesync/skills/
• guide agents to explain issue background and proposed solutions from issue details and comments
• handle missing issue arguments and unspecified solutions explicitly

Test plan

• pnpm cicheck

[Copilot]

Pull request overview

Adds a new official Rulesync skill (explain-issue) under .rulesync/skills/ to guide agents in summarizing a GitHub issue’s background and proposed solution, with explicit handling for missing issue input and unspecified solutions.

Changes:

• Introduce .rulesync/skills/explain-issue/SKILL.md defining the new explain-issue skill.
• Instruct agents to gather issue body + comments and produce a concise two-part explanation (background + proposed solution).
• Add explicit guidance for cases where the solution is not decided/recorded in the issue discussion.

[Copilot]

gh issue view <issue_number> --comments already outputs the issue body/metadata along with comments, so running both gh issue view <issue_number> and gh issue view <issue_number> --comments is redundant and doubles the GitHub API/CLI work. Consider collapsing this into a single gh issue view invocation (or using --json/--jq if you need specific fields) to keep the skill faster and simpler.

Suggested change

	Run the following in parallel:
	- Get the issue description and metadata: `gh issue view <issue_number>`
	- Get the issue comments: `gh issue view <issue_number> --comments`
	Run the following command:
	- Get the issue description, metadata, and comments: `gh issue view <issue_number> --comments`

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

[Copilot]

PR description mentions documenting a maintainer preference for Japanese responses in .github/copilot-instructions.md, but that change doesn’t appear to be included in this PR. Either add the corresponding update to .github/copilot-instructions.md or adjust the PR description to match the actual changes.

[dyoshikawa]

@dyoshikawa Thank you!