Add Cloud Agent development environment for caret

[oujonsin]

Summary

Adds a repository-managed Cloud Agent development environment so the caret R package (source in pkg/caret) is fully buildable, loadable, and testable in a fresh VM. Because .cursor/environment.json is committed, it becomes the highest-precedence environment source and activates for future Cloud Agents once merged — no dashboard Save required.

What's included

• .cursor/environment.json — declares the environment and its idempotent install command. No start/terminals since caret is a library, not a service.
• .cursor/install.sh — a self-contained, idempotent bootstrap that:
  1. Installs the R 4.6 toolchain + system build libraries from the CRAN apt repo (guarded so it's a fast no-op when R is already present).
  2. Configures the Posit Public Package Manager binary mirror for fast dependency installs, and sets legacy-compat compiler flags.
  3. Installs caret's runtime dependencies plus the model/test packages declared by the project's CI (.travis.yml).
  4. Builds and installs the in-repo caret package and verifies it loads.

Notable detail

caret's src/caret.c (derived from the legacy class package) uses the Sint type and DOUBLE_XMAX constant, both removed from modern R headers. Rather than modifying package source, the environment restores their historical definitions (Sint == int, DOUBLE_XMAX == DBL_MAX) via a ~/.R/Makevars compiler flag, keeping the repository pristine.

Validation

• R CMD INSTALL pkg/caret succeeds; library(caret) loads v6.0-80.
• End-to-end train()/predict(): knn 5-fold CV on iris (best k=7, 97.3% accuracy, exercising the compiled C code) and rpart regression on mtcars.
• testthat suite: 75 assertions pass, 32 skip, 4 fail. The 4 failures are pre-existing incompatibilities between the 2018-era caret and current CRAN (recipes bake() semantics, factor preservation, pROC numeric tolerance) — not environment defects.
• install.sh is idempotent (runs cleanly repeatedly).
• Verified via Cloud Agent environment builds:
  • Snapshot-backed build (bld-...07c5b9d1) — booted in a fresh agent: R 4.6.1, caret loads, all CI deps present, 75/4/32 tests.
  • Cold build with no snapshot (bld-...88558c1e) — proves the committed config is self-contained: installs R from scratch on the default base image and builds caret successfully (exit 0).