chore(llm): Add triage-issue skill

[chargome]

• 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)

Closes #19357 (added automatically)