feat: wire skills.external_dirs into all remaining discovery paths

The config key skills.external_dirs and core resolution (get_all_skills_dirs,
get_external_skills_dirs in agent/skill_utils.py) already existed but several
code paths still only scanned SKILLS_DIR. Now external dirs are respected
everywhere:

- skills_categories(): scan all dirs for category discovery
- _get_category_from_path(): resolve categories against any skills root
- skill_manager_tool._find_skill(): search all dirs for edit/patch/delete
- credential_files.get_skills_directory_mount(): mount all dirs into
  Docker/Singularity containers (external dirs at external_skills/<idx>)
- credential_files.iter_skills_files(): list files from all dirs for
  Modal/Daytona upload
- tools/environments/ssh.py: rsync all skill dirs to remote hosts
- gateway _check_unavailable_skill(): check disabled skills across all dirs

Usage in config.yaml:
  skills:
    external_dirs:
      - ~/repos/agent-skills/hermes
      - /shared/team-skills

tools/environments/singularity.py

@@ -265,8 +265,7 @@ class SingularityEnvironment(BaseEnvironment):
                     mount_entry["host_path"],
                     mount_entry["container_path"],
                 )
-            skills_mount = get_skills_directory_mount()
-            if skills_mount:
+            for skills_mount in get_skills_directory_mount():
                 cmd.extend(["--bind", f"{skills_mount['host_path']}:{skills_mount['container_path']}:ro"])
                 logger.info(
                     "Singularity: binding skills dir %s -> %s",