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/credential_files.py

@@ -193,8 +193,8 @@ def get_credential_file_mounts() -> List[Dict[str, str]]:
 
 def get_skills_directory_mount(
     container_base: str = "/root/.hermes",
-) -> Dict[str, str] | None:
-    """Return mount info for a symlink-safe copy of the skills directory.
+) -> list[Dict[str, str]]:
+    """Return mount info for all skill directories (local + external).
 
     Skills may include ``scripts/``, ``templates/``, and ``references/``
     subdirectories that the agent needs to execute inside remote sandboxes.
@@ -206,18 +206,34 @@ def get_skills_directory_mount(
     symlinks are present (the common case), the original directory is returned
     directly with zero overhead.
 
-    Returns a dict with ``host_path`` and ``container_path`` keys, or None.
+    Returns a list of dicts with ``host_path`` and ``container_path`` keys.
+    The local skills dir mounts at ``<container_base>/skills``, external dirs
+    at ``<container_base>/external_skills/<index>``.
     """
+    mounts = []
     hermes_home = _resolve_hermes_home()
     skills_dir = hermes_home / "skills"
-    if not skills_dir.is_dir():
-        return None
+    if skills_dir.is_dir():
+        host_path = _safe_skills_path(skills_dir)
+        mounts.append({
+            "host_path": host_path,
+            "container_path": f"{container_base.rstrip('/')}/skills",
+        })
 
-    host_path = _safe_skills_path(skills_dir)
-    return {
-        "host_path": host_path,
-        "container_path": f"{container_base.rstrip('/')}/skills",
-    }
+    # Mount external skill dirs
+    try:
+        from agent.skill_utils import get_external_skills_dirs
+        for idx, ext_dir in enumerate(get_external_skills_dirs()):
+            if ext_dir.is_dir():
+                host_path = _safe_skills_path(ext_dir)
+                mounts.append({
+                    "host_path": host_path,
+                    "container_path": f"{container_base.rstrip('/')}/external_skills/{idx}",
+                })
+    except ImportError:
+        pass
+
+    return mounts
 
 
 _safe_skills_tempdir: Path | None = None
@@ -271,24 +287,44 @@ def iter_skills_files(
 ) -> List[Dict[str, str]]:
     """Yield individual (host_path, container_path) entries for skills files.
 
-    Skips symlinks entirely.  Preferred for backends that upload files
-    individually (Daytona, Modal) rather than mounting a directory.
+    Includes both the local skills dir and any external dirs configured via
+    skills.external_dirs.  Skips symlinks entirely.  Preferred for backends
+    that upload files individually (Daytona, Modal) rather than mounting a
+    directory.
     """
+    result: List[Dict[str, str]] = []
+
     hermes_home = _resolve_hermes_home()
     skills_dir = hermes_home / "skills"
-    if not skills_dir.is_dir():
-        return []
+    if skills_dir.is_dir():
+        container_root = f"{container_base.rstrip('/')}/skills"
+        for item in skills_dir.rglob("*"):
+            if item.is_symlink() or not item.is_file():
+                continue
+            rel = item.relative_to(skills_dir)
+            result.append({
+                "host_path": str(item),
+                "container_path": f"{container_root}/{rel}",
+            })
+
+    # Include external skill dirs
+    try:
+        from agent.skill_utils import get_external_skills_dirs
+        for idx, ext_dir in enumerate(get_external_skills_dirs()):
+            if not ext_dir.is_dir():
+                continue
+            container_root = f"{container_base.rstrip('/')}/external_skills/{idx}"
+            for item in ext_dir.rglob("*"):
+                if item.is_symlink() or not item.is_file():
+                    continue
+                rel = item.relative_to(ext_dir)
+                result.append({
+                    "host_path": str(item),
+                    "container_path": f"{container_root}/{rel}",
+                })
+    except ImportError:
+        pass
 
-    container_root = f"{container_base.rstrip('/')}/skills"
-    result: List[Dict[str, str]] = []
-    for item in skills_dir.rglob("*"):
-        if item.is_symlink() or not item.is_file():
-            continue
-        rel = item.relative_to(skills_dir)
-        result.append({
-            "host_path": str(item),
-            "container_path": f"{container_root}/{rel}",
-        })
     return result