docs: environment setup + R env scripts

[DankerMu]

Summary

• Add project-local R dependency install/check scripts (tools/r/*) targeting .Rlib/.
• Add environment setup doc for macOS/Linux (docs/environment.md).
• Ignore .Rlib/ in .gitignore to prevent accidental commits.

Tests

• Rscript tools/r/check_env.R --skip-rshud --skip-lwgeom (PASS)
• Rscript tools/r/install_deps.R --skip-rshud --skip-lwgeom (PASS)

Notes:

• Full Rscript tools/r/check_env.R currently fails until rSHUD is installable without legacy deps (tracked in rSHUD#1).

Closes #20

[DankerMu]

Review: spec-driven-workflow (via codeagent)

Review summary

• Scope is tight: adds docs/environment.md, tools/r/install_deps.R, tools/r/check_env.R, and ignores .Rlib/.
• Scripts are readable/fail-fast, and the doc covers system deps + troubleshooting.

Issues

• [MEDIUM] Spec hygiene: this PR effectively implements “4. Environment & tooling (SHUD-NC)” in openspec/changes/refactor-r-spatial-stack-sf-terra/tasks.md, but the checklist isn’t updated. Consider checking off the completed items.
• [LOW] If users run tools/r/check_env.R with --lib, consider echoing the resolved lib path and including --lib in the suggested “Fix” commands.
• [LOW] .gitignore uses .Rlib/ (matches any .Rlib), which is fine, but if you want repo-root only use /.Rlib/.

Decision
PASS — score 4/5.