chore: add Cloud Agent development environment

[Sign312]

Summary

Adds a Cloud Agent development environment for this repository (CountDown.js), a dependency-free vanilla JavaScript countdown timer library.

Since the project has no package.json, dependencies, or build step (the minified dist/CountDown.min.js is committed), the environment is intentionally minimal:

• Uses Cursor's default base image (already ships Node 22 and Python 3.12).
• install: no-op message — nothing to install or build.
• terminals: starts a static HTTP server (python3 -m http.server 8000) so the demo page is reachable at http://localhost:8000/test.html.
• ports: exposes port 8000.

Validation

• install command runs successfully and is idempotent (ran twice).
• .cursor/environment.json validated against the environment schema.
• Static server serves test.html (HTTP 200) and dist/CountDown.min.js (HTTP 200).
• Node sanity check of the library's pure formatting functions: makeTimeText → 01:02:05, makeDateAndTimeText → 2天 03:04:09, all public APIs present.
• End-to-end browser demo of the countdown ticking down, the soon alert at 10s, and pause/resume/end controls all working (video below).
• Draft environment build bld-20260924-1eaf70ee-6c13-4cd3-b0bc-379787298968 SUCCEEDED, and a fresh Cloud Agent booted from that exact build passed all checks (tool versions, checkout, files present, install idempotence, static server HTTP 200, Node product action).

countdown_timer_pause_resume_end_demo.mp4

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