docs: update meta files for Rust-side CLAUDE* env var stripping

- .claude/CLAUDE.md: document dual-layer env var stripping (Rust + JS)
- docs/progress.md: add session entry for Rust-side stripping
- docs/task_plan.md: add CLAUDE* env var leak to errors table
- CHANGELOG.md: add fix entry under Unreleased
- .gitignore: exclude debug/, plugins/, projects/ (Claude Code working dirs)

.gitignore

@@ -3,3 +3,6 @@ __pycache__/
 *.pyo
 /CLAUDE.md
 v2/target/
+debug/
+plugins/
+projects/