Add AGENTS.md with Cursor Cloud development instructions

[n2parko]

Adds AGENTS.md with Cursor Cloud-specific development instructions for future cloud agents.

What this adds

• AGENTS.md documenting the two executable components in this content-first monorepo:
  1. Plugin validation — node scripts/validate-plugins.mjs (root level, requires ajv/ajv-formats)
  2. Orchestrate CLI — Bun-based TypeScript CLI with lint/typecheck/test commands
• Non-obvious gotchas (no root package.json, ad-hoc dependency install, mocked API keys in tests, expected Slack warnings)

Environment verification

All checks pass:

Check	Command	Result
Plugin validation	node scripts/validate-plugins.mjs	All plugins validated successfully
Orchestrate lint	bun run lint (Biome)	58 files checked, no issues
Orchestrate typecheck	tsc --noEmit	Clean
Orchestrate tests	bun test	207 pass, 0 fail

Test output:
orchestrate_test_output.log

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