fix(tier2): override top-level model to MiniMax-M3

The clone's opencode.json inherited the main repo's top-level 'model'
field (zai/glm-5) via 'git clone'. The tier2-autonomous agent has its
own 'model: minimax-coding-plan/MiniMax-M3' override, so the default
agent path was technically correct, but any other agent spawned without
an explicit model (or if the user manually switched to build/plan)
would have used zai/glm-5 instead of MiniMax-M3.

Fix:
1. Add top-level 'model: minimax-coding-plan/MiniMax-M3' to
   conductor/tier2/opencode.json.fragment.
2. setup_tier2_clone.ps1 merge now overrides 'model' from the fragment
   (was only overriding agent, permission, default_agent).
3. Added test_config_fragment_has_top_level_model (default-on) to
   assert the fragment's model field.
4. Added test_setup_script_overrides_model (opt-in TIER2_SANDBOX_TESTS=1)
   to assert the merge code.

All 17 tests pass (14 default-on + 3 opt-in).

Verified: re-ran setup against the live clone; opencode.json's
top-level 'model' is now minimax-coding-plan/MiniMax-M3.

tests/test_tier2_setup_bootstrap.py

@@ -52,3 +52,12 @@ def test_setup_script_overrides_mcp_server() -> None:
  assert "mcp.'manual-slop'.environment.PYTHONPATH" in content, "script must override MCP server PYTHONPATH"
  assert "mcp_paths.toml" in content, "script must reset mcp_paths.toml"
  assert "extra_dirs = []" in content, "script must set extra_dirs to empty"
+
+
+def test_setup_script_overrides_model() -> None:
+ """Regression test: setup_tier2_clone.ps1 MUST override the top-level
+ 'model' field in the clone's opencode.json to the Tier 2 model
+ (minimax-coding-plan/MiniMax-M3), not the main repo's zai/glm-5."""
+ script = Path("scripts/tier2/setup_tier2_clone.ps1").resolve()
+ content = script.read_text(encoding="utf-8")
+ assert 'Name model -Value $fragment.model' in content, "script must override top-level model from fragment"