refactor(mcp_tool_specs): delete redundant AGENT_TOOL_NAMES; use tool_names() at consumer sites

AGENT_TOOL_NAMES was a hardcoded snapshot of mcp_tool_specs.tool_names()
in src/models.py. The pre-existing test
test_tool_names_subset_of_models_agent_tool_names literally asserted
'tool_names() ⊆ AGENT_TOOL_NAMES' (proving the redundancy), and
AGENT_TOOL_NAMES was not maintained in lockstep with the registry
(it would silently drift if a new tool was added).

This commit:
 1. Deletes AGENT_TOOL_NAMES from src/models.py (replaced by an
    explanatory comment in the Constants section).
 2. Updates 3 consumer sites in src/app_controller.py:
    - 'for t in models.AGENT_TOOL_NAMES' -> 'for t in mcp_tool_specs.tool_names()'
    - (in 2 methods: __init__ + a setter)
 3. Updates 2 test sites in tests/test_arch_boundary_phase2.py:
    - 'from src.models import AGENT_TOOL_NAMES' -> 'from src import mcp_tool_specs'
    - 'AGENT_TOOL_NAMES' references -> 'mcp_tool_specs.tool_names()'
 4. Removes the tautology test
    test_tool_names_subset_of_models_agent_tool_names from
    tests/test_mcp_tool_specs.py (it asserted 'AGENT_TOOL_NAMES
    superset of tool_names()' which becomes meaningless after
    AGENT_TOOL_NAMES is deleted). Also removes the now-unused
    'from src import models' import from that test file.

Verification: VC9
  git grep 'AGENT_TOOL_NAMES' -- 'src/*.py' 'tests/*.py'  # 0 hits
  from src import mcp_tool_specs
  mcp_tool_specs.tool_names()  # returns the canonical 45 tools
  from src.app_controller import AppController  # uses the new path

Tests verified (15/16 PASS; 1 pre-existing failure unrelated to this
commit):
  tests/test_arch_boundary_phase2.py (6 tests; 1 pre-existing
                                          failure: test_rejection_prevents_dispatch
                                          is a dialog-mock issue that
                                          predates Phase 4)
  tests/test_mcp_tool_specs.py (10 tests; the tautology test was removed;
                                          the remaining 10 pass)

tests/test_mcp_tool_specs.py

@@ -7,7 +7,6 @@ Phase 1 of any_type_componentization_20260621. Verifies:
 - get_tool_schemas() returns the expected list
 - ToolParameter / ToolSpec dataclasses have correct frozen=True semantics
 - to_dict() round-trip preserves the legacy dict shape
-- Cross-module invariant: tool_names() == models.AGENT_TOOL_NAMES subset
 
 CONVENTION: 1-space indentation. NO COMMENTS.
 """
@@ -15,7 +14,6 @@ from __future__ import annotations
 
 import pytest
 from src import mcp_tool_specs
-from src import models
 
 
 EXPECTED_TOOLS: set[str] = {
@@ -107,14 +105,6 @@ def test_tool_parameter_to_dict_includes_enum() -> None:
  assert 'before' in d['enum']
 
 
-def test_tool_names_subset_of_models_agent_tool_names() -> None:
- """Cross-module invariant: every MCP tool is also an agent tool."""
- native_names = mcp_tool_specs.tool_names()
- agent_names = set(models.AGENT_TOOL_NAMES)
- missing_in_agent = native_names - agent_names
- assert not missing_in_agent, f"Native tools not in AGENT_TOOL_NAMES: {missing_in_agent}"
-
-
 def test_register_idempotent_replaces_existing() -> None:
  """register() should overwrite (idempotent for hot-reload scenarios)."""
  from src.mcp_tool_specs import ToolSpec, ToolParameter, register