fix: force flag passthrough, cross-platform hashes, manifest retention, docstring accuracy

- agent_switch: pass force=force (user's actual flag) instead of
  force=True so hash-check protection is preserved for unconfirmed files
- _hash_file_list: use as_posix() for POSIX-stable manifest keys;
  guard relative_to with try/except to skip files outside project root
- remove_tracked_files: updated docstring to accurately describe hash
  comparison behavior (values ARE used, not ignored); manifest is only
  deleted when all tracked files were removed (preserves tracking of
  skipped modified files)

src/specify_cli/__init__.py

@@ -2685,7 +2685,7 @@ def agent_switch(
                 console.print(f"  [dim]Tearing down {current_agent}...[/dim]")
                 current_bootstrap.teardown(
                     project_path,
-                    force=True,  # already confirmed above
+                    force=force,
                     files=all_files,
                 )
                 console.print(f"  [green]✓[/green] {current_agent} removed")