refactor(ai_client): move DEFAULT_TOOL_CATEGORIES from models.py to ai_client.py

Per post_module_taxonomy_de_cruft_20260627 Phase 3 (FR6). The
DEFAULT_TOOL_CATEGORIES constant groups the canonical MCP tool list
for the UI's category filter. The AI client is the natural owner
(it owns the tool spec registry via src.mcp_tool_specs); models.py
is a data-class shim, not a UI-config registry.

This commit:
 1. Adds DEFAULT_TOOL_CATEGORIES (the 7-category dict) to src/ai_client.py
    after the PROVIDERS constant. The dict is identical to the one that
    was in models.py.
 2. Updates src/gui_2.py (the single consumer) to:
    - Add 'from src.ai_client import DEFAULT_TOOL_CATEGORIES' to the
      import block
    - Replace all 6 'models.DEFAULT_TOOL_CATEGORIES' references with
      the bare 'DEFAULT_TOOL_CATEGORIES' name
 3. Removes the DEFAULT_TOOL_CATEGORIES dict from src/models.py
    (it was already removed as a side effect of the Phase 2.3
    __getattr__ removal commit; the file is now 70 lines).

The fix was performed by the one-time script
scripts/tier2/artifacts/post_module_taxonomy_de_cruft_20260627/fix_gui2_dtc.py
which does an in-place re.sub on src/gui_2.py.

Verification:
 - 'from src.ai_client import DEFAULT_TOOL_CATEGORIES' works
 - 'from src.models import DEFAULT_TOOL_CATEGORIES' raises ImportError
   (correctly; the constant moved)
 - All 7 references in src/gui_2.py resolve to the ai_client version
 - 'from src.models import Metadata' still returns TrackMetadata
   (the legacy alias is preserved)

src/gui_2.py

@@ -112,6 +112,7 @@ from src import log_registry
 # from src import log_pruner
 from src import models
 from src.models import GenerateRequest, ConfirmRequest
+from src.ai_client import DEFAULT_TOOL_CATEGORIES
 from src import mcp_client
 from src import markdown_helper
 from src import synthesis_formatter
@@ -2741,7 +2742,7 @@ def render_agent_tools_panel(app: App) -> None:
    imgui.end_combo()
   
   imgui.dummy(imgui.ImVec2(0, 8))
-  cat_options = ["All"] + sorted(list(models.DEFAULT_TOOL_CATEGORIES.keys()))
+  cat_options = ["All"] + sorted(list(DEFAULT_TOOL_CATEGORIES.keys()))
   #NOTE(Ed): Exception(Thirdparty)
   try:
    f_idx = cat_options.index(app.ui_tool_filter_category)
@@ -3231,7 +3232,7 @@ def render_tool_preset_manager_content(app: App, is_embedded: bool = False) -> N
   imgui.begin_child("tp_list_pane", imgui.ImVec2(0, 0), False)
   if True:
    if imgui.button("New Preset", imgui.ImVec2(-1, 0)):
-    app._editing_tool_preset_name = ""; app._editing_tool_preset_categories = {cat: {} for cat in models.DEFAULT_TOOL_CATEGORIES}
+    app._editing_tool_preset_name = ""; app._editing_tool_preset_categories = {cat: {} for cat in DEFAULT_TOOL_CATEGORIES}
     app._editing_tool_preset_scope = "project"; app._selected_tool_preset_idx = -1
    imgui.separator()
    preset_names = sorted(app.controller.tool_presets.keys())
@@ -3270,12 +3271,12 @@ def render_tool_preset_manager_content(app: App, is_embedded: bool = False) -> N
    if opened_t != app._tool_list_open: app._tool_list_open = opened_t
    if app._tool_list_open:
     imgui.text("Filter:"); imgui.same_line()
-    cat_opts = ["All"] + sorted(list(models.DEFAULT_TOOL_CATEGORIES.keys()))
+    cat_opts = ["All"] + sorted(list(DEFAULT_TOOL_CATEGORIES.keys()))
     f_idx    = cat_opts.index(app.ui_tool_filter_category) if app.ui_tool_filter_category in cat_opts else 0
     imgui.set_next_item_width(200); ch_cat, next_f_idx = imgui.combo("##tp_filter", f_idx, cat_opts)
     if ch_cat: app.ui_tool_filter_category = cat_opts[next_f_idx]
     with imscope.child("tp_scroll", 0, h1, True):
-     for cat_name, default_tools in models.DEFAULT_TOOL_CATEGORIES.items():
+     for cat_name, default_tools in DEFAULT_TOOL_CATEGORIES.items():
       if app.ui_tool_filter_category != "All" and app.ui_tool_filter_category != cat_name: continue
       if imgui.tree_node(cat_name):
        if cat_name not in app._editing_tool_preset_categories: app._editing_tool_preset_categories[cat_name] = []
@@ -3344,7 +3345,7 @@ def render_tool_preset_manager_content(app: App, is_embedded: bool = False) -> N
        if app._bias_weights_open:
         imgui.begin_child("btool_scroll", imgui.ImVec2(0, bh1), True)
         if True:
-         for cat_name, default_tools in models.DEFAULT_TOOL_CATEGORIES.items():
+         for cat_name, default_tools in DEFAULT_TOOL_CATEGORIES.items():
           if imgui.tree_node(f"{cat_name}##b_list"):
            if imgui.begin_table(f"bt_{cat_name}", 2):
             imgui.table_setup_column("T", imgui.TableColumnFlags_.width_fixed, 220)
@@ -3371,7 +3372,7 @@ def render_tool_preset_manager_content(app: App, is_embedded: bool = False) -> N
          if imgui.begin_table("bcats", 2):
           imgui.table_setup_column("C", imgui.TableColumnFlags_.width_fixed, 220)
           imgui.table_setup_column("M", imgui.TableColumnFlags_.width_stretch)
-          for cn in sorted(models.DEFAULT_TOOL_CATEGORIES.keys()):
+          for cn in sorted(DEFAULT_TOOL_CATEGORIES.keys()):
            imgui.table_next_row(); imgui.table_next_column()
            imgui.text(cn);         imgui.table_next_column()
            curr_m    = app._editing_bias_profile_category_multipliers.get(cn, 1.0); imgui.set_next_item_width(-1)