test(agents): lock env in uninstall memory-digest tests (#263)

Two sync uninstall tests set TRACEDECAY_DATA_DIR via EnvVarGuard without
taking PROCESS_ENV_LOCK, racing every env-guarded test in the same
binary under plain parallel cargo test — most visibly
test_codex_install_exports_active_managed_skills, whose install read a
foreign profile root and exported no managed skills. nextest (one
process per test) never hits it, so CI stayed green while local runs
flaked. Take the lock like every other env-mutating test.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: ScriptedAlchemy

tests/agent_suite/agent_test.rs

@@ -4147,6 +4147,7 @@ fn test_claude_install_then_uninstall() {
 
 #[test]
 fn test_claude_uninstall_unrecords_memory_digest_target() {
+    let _env_lock = AGENT_ENV_LOCK.blocking_lock();
     let dir = TempDir::new().unwrap();
     let home = dir.path();
     let profile_root = home.join(".tracedecay");
@@ -4270,6 +4271,7 @@ fn test_codex_install_then_uninstall() {
 
 #[test]
 fn test_codex_local_uninstall_unrecords_legacy_repo_memory_digest_target() {
+    let _env_lock = AGENT_ENV_LOCK.blocking_lock();
     let home = TempDir::new().unwrap();
     let project = TempDir::new().unwrap();
     let mut ctx = make_install_ctx(home.path());