fix(tui): @folder: only yields directories, @file: only yields files

Reported during TUI v2 blitz testing: typing `@folder:` in the composer
pulled up .dockerignore, .env, .gitignore, and every other file in the
cwd alongside the actual directories. The completion loop yielded every
entry regardless of the explicit prefix and auto-rewrote each completion
to @file: vs @folder: based on is_dir — defeating the user's choice.

Also fixed a pre-existing adjacent bug: a bare `@file:` or `@folder:`
(no path) used expanded=="." as both search_dir AND match_prefix,
filtering the list to dotfiles only. When expanded is empty or ".",
search in cwd with no prefix filter.

- want_dir = prefix == "@folder:" drives an explicit is_dir filter
- preserve the typed prefix in completion text instead of rewriting
- three regression tests cover: folder-only, file-only, and the bare-
  prefix case where completions keep the `@folder:` prefix

tui_gateway/server.py

@@ -2417,15 +2417,22 @@ def _(rid, params: dict) -> dict:
             ]
             return _ok(rid, {"items": items})
 
-        if is_context and query.startswith(("file:", "folder:")):
-            prefix_tag = query.split(":", 1)[0]
-            path_part = query.split(":", 1)[1] or "."
+        # Accept both `@folder:path` and the bare `@folder` form so the user
+        # sees directory listings as soon as they finish typing the keyword,
+        # without first accepting the static `@folder:` hint.
+        if is_context and query in ("file", "folder"):
+            prefix_tag, path_part = query, ""
+        elif is_context and query.startswith(("file:", "folder:")):
+            prefix_tag, _, tail = query.partition(":")
+            path_part = tail
         else:
             prefix_tag = ""
-            path_part = query if not is_context else query
+            path_part = query if is_context else query
 
-        expanded = _normalize_completion_path(path_part)
-        if expanded.endswith("/"):
+        expanded = _normalize_completion_path(path_part) if path_part else "."
+        if expanded == "." or not expanded:
+            search_dir, match = ".", ""
+        elif expanded.endswith("/"):
             search_dir, match = expanded, ""
         else:
             search_dir = os.path.dirname(expanded) or "."
@@ -2434,6 +2441,7 @@ def _(rid, params: dict) -> dict:
         if not os.path.isdir(search_dir):
             return _ok(rid, {"items": []})
 
+        want_dir = prefix_tag == "folder"
         match_lower = match.lower()
         for entry in sorted(os.listdir(search_dir)):
             if match and not entry.lower().startswith(match_lower):
@@ -2442,6 +2450,11 @@ def _(rid, params: dict) -> dict:
                 continue
             full = os.path.join(search_dir, entry)
             is_dir = os.path.isdir(full)
+            # Explicit `@folder:` / `@file:` — honour the user's filter.  Skip
+            # the opposite kind instead of auto-rewriting the completion tag,
+            # which used to defeat the prefix and let `@folder:` list files.
+            if prefix_tag and want_dir != is_dir:
+                continue
             rel = os.path.relpath(full)
             suffix = "/" if is_dir else ""