fix: remove /prompt slash command — footgun via prefix expansion (#6752)

/pr <anything> silently resolved to /prompt via the shortest-match
tiebreaker in prefix expansion, permanently overwriting the system
prompt and persisting to config. The command's functionality (setting
agent.system_prompt) is available via config.yaml and /personality
covers the common use case.

Removes: CommandDef, dispatch branch, _handle_prompt_command handler,
docs references, and updates subcommand extraction test.

tests/hermes_cli/test_commands.py

@@ -425,8 +425,8 @@ class TestSlashCommandCompleter:
 class TestSubcommands:
     def test_explicit_subcommands_extracted(self):
         """Commands with explicit subcommands on CommandDef are extracted."""
-        assert "/prompt" in SUBCOMMANDS
-        assert "clear" in SUBCOMMANDS["/prompt"]
+        assert "/skills" in SUBCOMMANDS
+        assert "install" in SUBCOMMANDS["/skills"]
 
     def test_reasoning_has_subcommands(self):
         assert "/reasoning" in SUBCOMMANDS