feat: add /plan command (#1372)

* feat: add /plan command

* refactor: back /plan with bundled skill

* docs: document /plan skill

tests/agent/test_skill_commands.py

@@ -1,13 +1,15 @@
 """Tests for agent/skill_commands.py — skill slash command scanning and platform filtering."""
 
 import os
+from datetime import datetime
 from unittest.mock import patch
 
 import tools.skills_tool as skills_tool_module
 from agent.skill_commands import (
-    scan_skill_commands,
-    build_skill_invocation_message,
+    build_plan_path,
     build_preloaded_skills_prompt,
+    build_skill_invocation_message,
+    scan_skill_commands,
 )
 
 
@@ -272,3 +274,35 @@ Generate some audio.
 
         assert msg is not None
         assert 'file_path="<path>"' in msg
+
+
+class TestPlanSkillHelpers:
+    def test_build_plan_path_uses_hermes_home_and_slugifies_request(self, tmp_path, monkeypatch):
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+
+        path = build_plan_path(
+            "Implement OAuth login + refresh tokens!",
+            now=datetime(2026, 3, 15, 9, 30, 45),
+        )
+
+        assert path == tmp_path / "plans" / "2026-03-15_093045-implement-oauth-login-refresh-tokens.md"
+
+    def test_plan_skill_message_can_include_runtime_save_path_note(self, tmp_path):
+        with patch("tools.skills_tool.SKILLS_DIR", tmp_path):
+            _make_skill(
+                tmp_path,
+                "plan",
+                body="Save plans under $HERMES_HOME/plans and do not execute the work.",
+            )
+            scan_skill_commands()
+            msg = build_skill_invocation_message(
+                "/plan",
+                "Add a /plan command",
+                runtime_note="Save the markdown plan with write_file to /tmp/plans/plan.md",
+            )
+
+        assert msg is not None
+        assert "Save plans under $HERMES_HOME/plans" in msg
+        assert "Add a /plan command" in msg
+        assert "/tmp/plans/plan.md" in msg
+        assert "Runtime note:" in msg