feat(skills): /reload-skills slash command + skills_reload agent tool

Adds a public reload path for the in-process skill caches so newly
installed (or removed) skills become visible mid-session without a
gateway restart. Mirrors the shape of /reload-mcp.

Three surfaces:
* /reload-skills slash command — CLI (cli.py) and gateway (gateway/run.py),
  with /reload_skills alias for Telegram autocomplete and an explicit
  Discord registration.
* skills_reload agent tool (tools/skills_tool.py) — lets agents/subagents
  pick up freshly-installed skills via tool call.
* agent.skill_commands.reload_skills() — shared helper that clears
  _skill_commands, _SKILLS_PROMPT_CACHE (in-process LRU), and the
  on-disk .skills_prompt_snapshot.json, then returns an added/removed
  diff plus the new total count.

Tested:
* tests/agent/test_skill_commands_reload.py (9 cases)
* tests/cli/test_cli_reload_skills.py       (3 cases)
* tests/gateway/test_reload_skills_command.py (4 cases)

Use case: NemoClaw / OpenShell-style sandboxed orchestrators that drop
skills into ~/.hermes/skills mid-session, plus agentic flows where the
agent itself installs a skill via the shell tool and needs it bound
without a gateway restart. The Python helper
clear_skills_system_prompt_cache(clear_snapshot=True) already exists
internally — this PR just exposes it via slash command and tool.

tests/gateway/test_reload_skills_command.py

@@ -0,0 +1,174 @@
+"""Tests for the ``/reload-skills`` gateway slash command handler.
+
+Verifies the gateway path that mirrors ``/reload-mcp``:
+  * dispatcher routes ``/reload-skills`` to ``_handle_reload_skills_command``
+  * the underscored alias ``/reload_skills`` is not flagged as unknown
+  * the handler invokes ``agent.skill_commands.reload_skills`` and renders a
+    human-readable diff
+"""
+
+from datetime import datetime
+from types import SimpleNamespace
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from gateway.config import GatewayConfig, Platform, PlatformConfig
+from gateway.platforms.base import MessageEvent
+from gateway.session import SessionEntry, SessionSource, build_session_key
+
+
+def _make_source() -> SessionSource:
+    return SessionSource(
+        platform=Platform.TELEGRAM,
+        user_id="u1",
+        chat_id="c1",
+        user_name="tester",
+        chat_type="dm",
+    )
+
+
+def _make_event(text: str) -> MessageEvent:
+    return MessageEvent(text=text, source=_make_source(), message_id="m1")
+
+
+def _make_runner():
+    from gateway.run import GatewayRunner
+
+    runner = object.__new__(GatewayRunner)
+    runner.config = GatewayConfig(
+        platforms={Platform.TELEGRAM: PlatformConfig(enabled=True, token="***")}
+    )
+    adapter = MagicMock()
+    adapter.send = AsyncMock()
+    runner.adapters = {Platform.TELEGRAM: adapter}
+    runner._voice_mode = {}
+    runner.hooks = SimpleNamespace(
+        emit=AsyncMock(),
+        emit_collect=AsyncMock(return_value=[]),
+        loaded_hooks=False,
+    )
+
+    session_entry = SessionEntry(
+        session_key=build_session_key(_make_source()),
+        session_id="sess-1",
+        created_at=datetime.now(),
+        updated_at=datetime.now(),
+        platform=Platform.TELEGRAM,
+        chat_type="dm",
+    )
+    runner.session_store = MagicMock()
+    runner.session_store.get_or_create_session.return_value = session_entry
+    runner.session_store.load_transcript.return_value = []
+    runner.session_store.has_any_sessions.return_value = True
+    runner.session_store.append_to_transcript = MagicMock()
+    runner.session_store.rewrite_transcript = MagicMock()
+    runner.session_store.update_session = MagicMock()
+    runner._running_agents = {}
+    runner._pending_messages = {}
+    runner._pending_approvals = {}
+    runner._session_db = None
+    runner._reasoning_config = None
+    runner._provider_routing = {}
+    runner._fallback_model = None
+    runner._show_reasoning = False
+    runner._is_user_authorized = lambda _source: True
+    runner._set_session_env = lambda _context: None
+    runner._should_send_voice_reply = lambda *_args, **_kwargs: False
+    return runner
+
+
+@pytest.mark.asyncio
+async def test_reload_skills_handler_renders_added_and_removed(monkeypatch):
+    """The handler should call ``reload_skills`` and surface the diff."""
+    import gateway.run as gateway_run
+
+    fake_result = {
+        "added": ["alpha", "beta"],
+        "removed": ["gamma"],
+        "unchanged": ["delta"],
+        "total": 3,
+        "commands": 3,
+    }
+
+    def _fake_reload_skills():
+        return fake_result
+
+    # Patch the symbol where ``_handle_reload_skills_command`` imports it from.
+    import agent.skill_commands as skill_commands_mod
+    monkeypatch.setattr(skill_commands_mod, "reload_skills", _fake_reload_skills)
+
+    runner = _make_runner()
+    out = await runner._handle_reload_skills_command(_make_event("/reload-skills"))
+
+    assert out is not None
+    assert "Skills Reloaded" in out
+    assert "alpha" in out and "beta" in out
+    assert "gamma" in out
+    assert "3 skill(s) available" in out
+
+    # A history note should be appended so the model sees the diff next turn.
+    runner.session_store.append_to_transcript.assert_called_once()
+    appended = runner.session_store.append_to_transcript.call_args[0][1]
+    assert appended["role"] == "user"
+    assert "Skills have been reloaded" in appended["content"]
+
+
+@pytest.mark.asyncio
+async def test_reload_skills_handler_reports_no_changes(monkeypatch):
+    """When nothing changed, the handler should say so without injecting a note."""
+    import agent.skill_commands as skill_commands_mod
+
+    monkeypatch.setattr(
+        skill_commands_mod,
+        "reload_skills",
+        lambda: {
+            "added": [],
+            "removed": [],
+            "unchanged": ["alpha"],
+            "total": 1,
+            "commands": 1,
+        },
+    )
+
+    runner = _make_runner()
+    out = await runner._handle_reload_skills_command(_make_event("/reload-skills"))
+
+    assert "No changes detected" in out
+    assert "1 skill(s) available" in out
+    # No history note when nothing changed — preserves prompt cache.
+    runner.session_store.append_to_transcript.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_dispatcher_routes_reload_skills(monkeypatch):
+    """``/reload-skills`` must reach ``_handle_reload_skills_command``."""
+    import gateway.run as gateway_run
+
+    runner = _make_runner()
+    sentinel = "reload-skills handler reached"
+    runner._handle_reload_skills_command = AsyncMock(return_value=sentinel)  # type: ignore[attr-defined]
+
+    monkeypatch.setattr(
+        gateway_run, "_resolve_runtime_agent_kwargs", lambda: {"api_key": "***"}
+    )
+
+    result = await runner._handle_message(_make_event("/reload-skills"))
+    assert result == sentinel
+
+
+@pytest.mark.asyncio
+async def test_underscored_alias_not_flagged_unknown(monkeypatch):
+    """Telegram autocomplete sends ``/reload_skills`` for ``/reload-skills``."""
+    import gateway.run as gateway_run
+
+    runner = _make_runner()
+    runner._handle_reload_skills_command = AsyncMock(return_value="ok")  # type: ignore[attr-defined]
+
+    monkeypatch.setattr(
+        gateway_run, "_resolve_runtime_agent_kwargs", lambda: {"api_key": "***"}
+    )
+
+    result = await runner._handle_message(_make_event("/reload_skills"))
+    if result is not None:
+        assert "Unknown command" not in result