chore(llm): Add `triage-issue` skill

[github-actions]

Note

The pull request "chore(llm): Add triage-issue skill" was created by @chargome but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

• Adds a new local Claude Code skill at .claude/skills/triage-issue/SKILL.md
• Invoked via /triage-issue <issue-number-or-url> [--ci] to triage GitHub issues on getsentry/sentry-javascript
• Produces a structured report with classification, root cause analysis, cross-repo search results, and actionable next steps
• Optional --ci flag outputs a Linear payload stub (to be wired up later)

[linear]

JS-1731