fix: Docker backend fails when docker is not in PATH (macOS gateway)

On macOS, Docker Desktop installs the CLI to /usr/local/bin/docker, but
when Hermes runs as a gateway service (launchd) or in other non-login
contexts, /usr/local/bin is often not in PATH. This causes the Docker
requirements check to fail with 'No such file or directory: docker' even
though docker works fine from the user's terminal.

Add find_docker() helper that uses shutil.which() first, then probes
common Docker Desktop install paths on macOS (/usr/local/bin,
/opt/homebrew/bin, Docker.app bundle). The resolved path is cached and
passed to mini-swe-agent via its 'executable' parameter.

- tools/environments/docker.py: add find_docker(), use it in
  _storage_opt_supported() and pass to _Docker(executable=...)
- tools/terminal_tool.py: use find_docker() in requirements check
- tests/tools/test_docker_find.py: 4 tests (PATH, fallback, not found, cache)

2877 tests pass.

tools/terminal_tool.py

@@ -1112,9 +1112,14 @@ def check_terminal_requirements() -> bool:
             return True
         elif env_type == "docker":
             from minisweagent.environments.docker import DockerEnvironment
-            # Check if docker is available
+            # Check if docker is available (use find_docker for macOS PATH issues)
+            from tools.environments.docker import find_docker
             import subprocess
-            result = subprocess.run(["docker", "version"], capture_output=True, timeout=5)
+            docker = find_docker()
+            if not docker:
+                logger.error("Docker executable not found in PATH or common install locations")
+                return False
+            result = subprocess.run([docker, "version"], capture_output=True, timeout=5)
             return result.returncode == 0
         elif env_type == "singularity":
             from minisweagent.environments.singularity import SingularityEnvironment