chore(agents): Migrate repo-wide cursor rules to skills #19550

• Create /release, /upgrade-dep, /upgrade-otel skills from .cursor/rules/ procedural guides
• Inline fetch-docs/attributes.mdc and fetch-docs/scopes.mdc as reference links in root AGENTS.md
• Delete 5 migrated .cursor/rules/ files (-250 lines of Cursor-specific format)

Some cursor rules used alwaysApply: false, meaning they loaded on-demand. Moving them into AGENTS.md (always loaded) would waste context tokens on every interaction. Skills are the right fit — they're on-demand, match the existing pattern (/e2e, /triage-issue, etc.), and work across all agents.

Part of the .cursor/rules/ → AGENTS.md migration. Remaining files(adding-a-new-ai-integration.mdc + browser fetch-docs) will be handled in a follow-up PR with scoped AGENTS.md files.