Add atomic writes, magic string constants, unit tests, type hints, and fix navigation

- save_json() now writes to a temp file then uses os.replace() for atomic writes
- Replace hardcoded "batch_data", "history_tree", "prompt_history", "sequence_number"
  strings with constants (KEY_BATCH_DATA, etc.) across all modules
- Add 29 unit tests for history_tree, utils, and json_loader
- Add type hints to public functions in utils.py, json_loader.py, history_tree.py
- Remove ALLOWED_BASE_DIR restriction that blocked navigating outside app CWD
- Fix path text input not updating on navigation by using session state key
- Add unpin button (❌) for removing pinned folders

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_history_tree.py

@@ -0,0 +1,67 @@
+import pytest
+from history_tree import HistoryTree
+
+
+def test_commit_creates_node_with_correct_parent():
+    tree = HistoryTree({})
+    id1 = tree.commit({"a": 1}, note="first")
+    id2 = tree.commit({"b": 2}, note="second")
+
+    assert tree.nodes[id1]["parent"] is None
+    assert tree.nodes[id2]["parent"] == id1
+
+
+def test_checkout_returns_correct_data():
+    tree = HistoryTree({})
+    id1 = tree.commit({"val": 42}, note="snap")
+    result = tree.checkout(id1)
+    assert result == {"val": 42}
+
+
+def test_checkout_nonexistent_returns_none():
+    tree = HistoryTree({})
+    assert tree.checkout("nonexistent") is None
+
+
+def test_cycle_detection_raises():
+    tree = HistoryTree({})
+    id1 = tree.commit({"a": 1})
+    # Manually introduce a cycle
+    tree.nodes[id1]["parent"] = id1
+    with pytest.raises(ValueError, match="Cycle detected"):
+        tree.commit({"b": 2})
+
+
+def test_branch_creation_on_detached_head():
+    tree = HistoryTree({})
+    id1 = tree.commit({"a": 1})
+    id2 = tree.commit({"b": 2})
+    # Detach head by checking out a non-tip node
+    tree.checkout(id1)
+    # head_id is now id1, which is no longer a branch tip (main points to id2)
+    id3 = tree.commit({"c": 3})
+    # A new branch should have been created
+    assert len(tree.branches) == 2
+    assert tree.nodes[id3]["parent"] == id1
+
+
+def test_legacy_migration():
+    legacy = {
+        "prompt_history": [
+            {"note": "Entry A", "seed": 1},
+            {"note": "Entry B", "seed": 2},
+        ]
+    }
+    tree = HistoryTree(legacy)
+    assert len(tree.nodes) == 2
+    assert tree.head_id is not None
+    assert tree.branches["main"] == tree.head_id
+
+
+def test_to_dict_roundtrip():
+    tree = HistoryTree({})
+    tree.commit({"x": 1}, note="test")
+    d = tree.to_dict()
+    tree2 = HistoryTree(d)
+    assert tree2.head_id == tree.head_id
+    assert tree2.nodes == tree.nodes