feat: ADR-0006 §8 step 6 — .snapshots/ gitignore + export + cleanup

The undo ring is local working state, handled at all three
project-file seams (R13):
- .gitignore template ignores /.snapshots/
- export excludes .snapshots/ (like playground.db / history.log)
- safely_delete_temp_project allowlists .snapshots/ so a temp that
  was modified then undone back to empty stays auto-deletable
- undo::SNAPSHOTS_DIR is now a pub const referenced by all three
- tests: gitignore content, export exclusion, cleanup allowlist

1693 passed / 0 failed / 1 ignored; clippy clean.

tests/project_lifecycle.rs

@@ -43,6 +43,8 @@ fn no_args_creates_temp_project_under_data_root() {
     // .gitignore must NOT include history.log (ADR-0007 amendment).
     let gi = fs::read_to_string(path.join(GITIGNORE)).unwrap();
     assert!(!gi.contains("history.log"));
+    // …but it must ignore the undo ring (ADR-0006 Amendment 1).
+    assert!(gi.contains("/.snapshots/"));
 }
 
 #[test]