fix: worktree-aware minisweagent path discovery + clean up requirements check (#1248)

Salvage of PR #1246 by ChatGPT (teknium1 session), resolved against
current main which already includes #1239.

Changes:
- Add minisweagent_path.py: worktree-aware helper that finds
  mini-swe-agent/src from either the current checkout or the main
  checkout behind a git worktree
- Use the helper in tools/terminal_tool.py and mini_swe_runner.py
  instead of naive path-relative lookup that fails in worktrees
- Clean up check_terminal_requirements():
  - local: return True (no minisweagent dep, per #1239)
  - singularity/ssh: remove unnecessary minisweagent imports
  - docker/modal: use importlib.util.find_spec with clear error
- Add regression tests for worktree path discovery and tool resolution

tests/tools/test_terminal_tool_requirements.py

@@ -0,0 +1,28 @@
+"""Tests for terminal/file tool availability in local dev environments."""
+
+import importlib
+
+from model_tools import get_tool_definitions
+
+terminal_tool_module = importlib.import_module("tools.terminal_tool")
+
+
+class TestTerminalRequirements:
+    def test_local_backend_does_not_require_minisweagent_package(self, monkeypatch):
+        monkeypatch.setattr(
+            terminal_tool_module,
+            "_get_env_config",
+            lambda: {"env_type": "local"},
+        )
+        assert terminal_tool_module.check_terminal_requirements() is True
+
+    def test_terminal_and_file_tools_resolve_for_local_backend(self, monkeypatch):
+        monkeypatch.setattr(
+            terminal_tool_module,
+            "_get_env_config",
+            lambda: {"env_type": "local"},
+        )
+        tools = get_tool_definitions(enabled_toolsets=["terminal", "file"], quiet_mode=True)
+        names = {tool["function"]["name"] for tool in tools}
+        assert "terminal" in names
+        assert {"read_file", "write_file", "patch", "search_files"}.issubset(names)