docs: add AGENTS.md with Cursor Cloud dev environment notes

[jqdsouza]

Summary

Sets up and verifies the development environment for this repo in Cursor Cloud, and records durable, non-obvious setup notes for future cloud agents in a new AGENTS.md.

No product/source code was changed — this PR only adds AGENTS.md. The reusable dependency-refresh steps are captured in the Cloud environment update script (installs Go 1.25, warms Go module + npm caches), not in the repo.

What was verified

• Go toolchain: base image ships Go 1.22 which is too old; installed Go 1.25.5 (satisfies go 1.24 modules and the go 1.25.0 collector exporter module). go.dev is egress-blocked, so GOTOOLCHAIN=auto fails — set GOTOOLCHAIN=local and fetch the toolchain from dl.google.com.
• Build: cd cli/beacon && make build (builds + embeds beacon-hooks).
• Go tests: go test ./... across all four modules + go test -race ./internal/endpoint/.... All pass except two macOS-only tests that CI runs on macos-latest (TestConfigureVSCodePreservesSettingsAndDisablesContentCaptureByDefault, TestRollbackReportsCollectorRestartFailure).
• Lint: golangci-lint run executes (not a CI gate; reports pre-existing findings).
• TypeScript SDK: npm ci, npm test (22 passing), npm run check, npm run build, npm run pack:dry-run.
• End-to-end product run: captured telemetry via the hooks binary (as Cursor does), detected a curl-pipe-to-shell threat with beacon scan, and viewed the events + findings in the local beacon endpoint dashboard.

beacon_dashboard_walkthrough.mp4

Findings page showing curl-pipe-to-shell HIGH finding

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