chore(agents): Consolidate SDK dev rules into `AGENTS.md`

[github-actions]

Note

The pull request "chore(agents): Consolidate SDK dev rules into AGENTS.md" was created by @chargome but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

• Create AGENTS.md as the single source of truth for SDK development rules
• Remove .cursor/rules/sdk_development.mdc (all content now in AGENTS.md)
• Slim CLAUDE.md down to just an @AGENTS.md reference

Both CLAUDE.md and .cursor/rules/sdk_development.mdc contained duplicated SDK development rules that could drift out of sync. AGENTS.md is the standard cross-tool format — Cursor, Claude Code, and other agents all read it automatically, so a single shared file eliminates duplication.

[linear]

JS-1807