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/ssh.py

@@ -135,9 +135,8 @@ class SSHEnvironment(PersistentShellMixin, BaseEnvironment):
                 else:
                     logger.debug("SSH: rsync credential failed: %s", result.stderr.strip())
 
-            # Sync skills directory (remap to detected home)
-            skills_mount = get_skills_directory_mount(container_base=container_base)
-            if skills_mount:
+            # Sync skill directories (local + external, remap to detected home)
+            for skills_mount in get_skills_directory_mount(container_base=container_base):
                 remote_path = skills_mount["container_path"]
                 mkdir_cmd = self._build_ssh_command()
                 mkdir_cmd.append(f"mkdir -p {remote_path}")