Adds WEEVIL_HOME-based test isolation so cargo test never touches
the real system. All commands run against a fresh TempDir per test.
Environment tests cover doctor, uninstall, new, and setup across
every combination of missing/present dependencies. Project lifecycle
tests cover creation, config persistence, upgrade, and build scripts.
Full round-trip lifecycle test: new → gradlew test → gradlew
compileJava → uninstall → doctor (unhealthy) → setup → doctor
(healthy). Confirms skeleton projects build and pass tests out of
the box, and that uninstall leaves user projects untouched.
34 tests, zero warnings.
d2cc62e32f