Fix remaining Python parity gaps with Go

Safety:
- sync.py: dirty-tree guard before rebase
- retarget.py: dirty-tree guard before rebase
- undo.py redo: dirty-tree guard with confirmation before git reset --hard

Correctness:
- undo.py: reflog loop uses continue instead of break (matches Go fix)

Cleanup:
- Deleted dead src/gx/ui/shelf_app.py (Textual TUI, no longer used)

Author: mubbie

src/gx/commands/retarget.py

@@ -22,6 +22,7 @@
     branch_exists,
     ensure_git_repo,
     get_current_branch,
+    is_clean_working_tree,
     run_git,
 )
 from gx.utils.stack import get_parent, get_parent_head, record_relationship
@@ -102,6 +103,10 @@ def retarget(
         ])
         return
 
+    if not is_clean_working_tree():
+        print_warning("You have uncommitted changes. Stash or commit them before retargeting.")
+        raise typer.Exit(1)
+
     if not confirm_action(f"Retarget {branch} onto {new_target}?"):
         print_info("Cancelled.")
         raise typer.Exit(0)

src/gx/commands/sync.py

@@ -21,6 +21,7 @@
     GitError,
     ensure_git_repo,
     get_current_branch,
+    is_clean_working_tree,
     run_git,
     supports_update_refs,
 )
@@ -230,6 +231,11 @@ def sync(
         print_dry_run(actions)
         return
 
+    # Check for uncommitted changes
+    if not is_clean_working_tree():
+        print_warning("You have uncommitted changes. Stash or commit them before syncing.")
+        raise typer.Exit(1)
+
     # Confirm for long chains
     if len(sync_branches) >= SYNC_CONFIRM_THRESHOLD:
         if not confirm_action(f"Sync {len(sync_branches)} branches?"):

src/gx/commands/undo.py

@@ -19,13 +19,15 @@
     print_info,
     print_success,
     print_table,
+    print_warning,
 )
 from gx.utils.git import (
     GitError,
     ensure_git_repo,
     get_last_commit,
     get_reflog_entries,
     get_repo_root,
+    is_clean_working_tree,
     run_git,
     time_ago,
 )
@@ -161,7 +163,7 @@ def _detect_state() -> dict | None:
                 "action_msg": "Soft reset to previous commit. Your changes will be preserved in staging.",
             }
 
-        break  # Only inspect the most recent meaningful action
+        continue  # Skip non-matching entries and check subsequent ones
 
     return None
 
@@ -294,6 +296,13 @@ def redo() -> None:
         print_info("Cancelled.")
         raise typer.Exit(0)
 
+    # Check for uncommitted changes before hard reset
+    if not is_clean_working_tree():
+        print_warning("You have uncommitted changes that would be lost by redo.")
+        if not confirm_action("Proceed anyway?"):
+            print_info("Cancelled.")
+            raise typer.Exit(0)
+
     # Redo by resetting to pre-state
     pre_ref = last_undo.get("pre_state_ref", "")
     if not pre_ref: