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.

website/docs/user-guide/features/skills.md

@@ -8,7 +8,9 @@ description: "On-demand knowledge documents — progressive disclosure, agent-ma
 
 Skills are on-demand knowledge documents the agent can load when needed. They follow a **progressive disclosure** pattern to minimize token usage and are compatible with the [agentskills.io](https://agentskills.io/specification) open standard.
 
-All skills live in **`~/.hermes/skills/`** — a single directory that serves as the source of truth. On fresh install, bundled skills are copied from the repo. Hub-installed and agent-created skills also go here. The agent can modify or delete any skill.
+All skills live in **`~/.hermes/skills/`** — the primary directory and source of truth. On fresh install, bundled skills are copied from the repo. Hub-installed and agent-created skills also go here. The agent can modify or delete any skill.
+
+You can also point Hermes at **external skill directories** — additional folders scanned alongside the local one. See [External Skill Directories](#external-skill-directories) below.
 
 See also:
 
@@ -164,6 +166,47 @@ Once set, declared env vars are **automatically passed through** to `execute_cod
 └── .bundled_manifest              # Tracks seeded bundled skills
 ```
 
+## External Skill Directories
+
+If you maintain skills outside of Hermes — for example, a shared `~/.agents/skills/` directory used by multiple AI tools — you can tell Hermes to scan those directories too.
+
+Add `external_dirs` under the `skills` section in `~/.hermes/config.yaml`:
+
+```yaml
+skills:
+  external_dirs:
+    - ~/.agents/skills
+    - /home/shared/team-skills
+    - ${SKILLS_REPO}/skills
+```
+
+Paths support `~` expansion and `${VAR}` environment variable substitution.
+
+### How it works
+
+- **Read-only**: External dirs are only scanned for skill discovery. When the agent creates or edits a skill, it always writes to `~/.hermes/skills/`.
+- **Local precedence**: If the same skill name exists in both the local dir and an external dir, the local version wins.
+- **Full integration**: External skills appear in the system prompt index, `skills_list`, `skill_view`, and as `/skill-name` slash commands — no different from local skills.
+- **Non-existent paths are silently skipped**: If a configured directory doesn't exist, Hermes ignores it without errors. Useful for optional shared directories that may not be present on every machine.
+
+### Example
+
+```text
+~/.hermes/skills/               # Local (primary, read-write)
+├── devops/deploy-k8s/
+│   └── SKILL.md
+└── mlops/axolotl/
+    └── SKILL.md
+
+~/.agents/skills/               # External (read-only, shared)
+├── my-custom-workflow/
+│   └── SKILL.md
+└── team-conventions/
+    └── SKILL.md
+```
+
+All four skills appear in your skill index. If you create a new skill called `my-custom-workflow` locally, it shadows the external version.
+
 ## Agent-Managed Skills (skill_manage tool)
 
 The agent can create, update, and delete its own skills via the `skill_manage` tool. This is the agent's **procedural memory** — when it figures out a non-trivial workflow, it saves the approach as a skill for future reuse.