feat(skills): support external skill directories via config (#3678)

Add skills.external_dirs config option — a list of additional directories
to scan for skills alongside ~/.hermes/skills/. External dirs are read-only:
skill creation/editing always writes to the local dir. Local skills take
precedence when names collide.

This lets users share skills across tools/agents without copying them into
Hermes's own directory (e.g. ~/.agents/skills, /shared/team-skills).

Changes:
- agent/skill_utils.py: add get_external_skills_dirs() and get_all_skills_dirs()
- agent/prompt_builder.py: scan external dirs in build_skills_system_prompt()
- tools/skills_tool.py: _find_all_skills() and skill_view() search external dirs;
  security check recognizes configured external dirs as trusted
- agent/skill_commands.py: /skill slash commands discover external skills
- hermes_cli/config.py: add skills.external_dirs to DEFAULT_CONFIG
- cli-config.yaml.example: document the option
- tests/agent/test_external_skills.py: 11 tests covering discovery, precedence,
  deduplication, and skill_view for external skills

Requested by community member primco.

hermes_cli/config.py

@@ -366,6 +366,13 @@ DEFAULT_CONFIG = {
     # Never saved to sessions, logs, or trajectories.
     "prefill_messages_file": "",
     
+    # Skills — external skill directories for sharing skills across tools/agents.
+    # Each path is expanded (~, ${VAR}) and resolved.  Read-only — skill creation
+    # always goes to ~/.hermes/skills/.
+    "skills": {
+        "external_dirs": [],   # e.g. ["~/.agents/skills", "/shared/team-skills"]
+    },
+
     # Honcho AI-native memory -- reads ~/.honcho/config.json as single source of truth.
     # This section is only needed for hermes-specific overrides; everything else
     # (apiKey, workspace, peerName, sessions, enabled) comes from the global config.