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.

src/undo.rs

@@ -49,7 +49,11 @@ use crate::project::{DATA_DIR, PLAYGROUND_DB, PROJECT_YAML};
 pub const DEFAULT_RING_CAPACITY: usize = 50;
 
 /// Directory under the project that holds the ring.
-const SNAPSHOTS_DIR: &str = ".snapshots";
+///
+/// Public so the `.gitignore` template, the export exclusion, and
+/// the temp-project cleanup allowlist all reference the one
+/// canonical name (ADR-0006 Amendment 1).
+pub const SNAPSHOTS_DIR: &str = ".snapshots";
 /// In-flight snapshot directory (one at a time).
 const STAGING_DIR: &str = ".staging";
 /// The ring index file.