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

mini_swe_runner.py

@@ -42,10 +42,11 @@ from dotenv import load_dotenv
 # Load environment variables
 load_dotenv()
 
-# Add mini-swe-agent to path if not installed
-mini_swe_path = Path(__file__).parent / "mini-swe-agent" / "src"
-if mini_swe_path.exists():
-    sys.path.insert(0, str(mini_swe_path))
+# Add mini-swe-agent to path if not installed. In git worktrees the populated
+# submodule may live in the main checkout rather than the worktree itself.
+from minisweagent_path import ensure_minisweagent_on_path
+
+ensure_minisweagent_on_path(Path(__file__).resolve().parent)
 
 
 # ============================================================================