fix: add prefix matching to slash command dispatcher

Slash commands previously required exact full names. Typing /con
returned 'Unknown command' even though /config was the only match.

Add unambiguous prefix matching in process_command():
- Unique prefix (e.g. /con -> /config): dispatch immediately
- Ambiguous prefix (e.g. /re -> /reset, /retry, /reasoning...):
  show 'Did you mean' suggestions
- No match: existing 'Unknown command' error

Prefix matching uses the COMMANDS dict from hermes_cli/commands.py
(same source as SlashCommandCompleter) so it stays in sync with
any new commands added there.

Closes #928

tests/test_cli_prefix_matching.py

@@ -0,0 +1,60 @@
+"""Tests for slash command prefix matching in HermesCLI.process_command."""
+from unittest.mock import MagicMock, patch
+from cli import HermesCLI
+
+
+def _make_cli():
+    cli_obj = HermesCLI.__new__(HermesCLI)
+    cli_obj.config = {}
+    cli_obj.console = MagicMock()
+    cli_obj.agent = None
+    cli_obj.conversation_history = []
+    return cli_obj
+
+
+class TestSlashCommandPrefixMatching:
+    def test_unique_prefix_dispatches_command(self):
+        """/con should dispatch to /config when it uniquely matches."""
+        cli_obj = _make_cli()
+        with patch.object(cli_obj, 'show_config') as mock_config:
+            cli_obj.process_command("/con")
+        mock_config.assert_called_once()
+
+    def test_unique_prefix_with_args_dispatches_command(self):
+        """/mo with argument should dispatch to /model."""
+        cli_obj = _make_cli()
+        with patch.object(cli_obj, 'process_command', wraps=cli_obj.process_command):
+            with patch("hermes_cli.models.fetch_api_models", return_value=None), \
+                 patch("cli.save_config_value"):
+                cli_obj.model = "current-model"
+                cli_obj.provider = "openrouter"
+                cli_obj.base_url = "https://openrouter.ai/api/v1"
+                cli_obj.api_key = "test"
+                cli_obj._explicit_api_key = None
+                cli_obj._explicit_base_url = None
+                cli_obj.requested_provider = "openrouter"
+                # /mod uniquely matches /model
+                result = cli_obj.process_command("/mod")
+        assert result is True
+
+    def test_ambiguous_prefix_shows_suggestions(self):
+        """/re matches /reset, /retry, /reload-mcp, /reasoning, /rollback — should show suggestions."""
+        cli_obj = _make_cli()
+        cli_obj.process_command("/re")
+        # Should print ambiguous message, not unknown command
+        printed = " ".join(str(c) for c in cli_obj.console.print.call_args_list)
+        assert "Ambiguous" in printed or "Did you mean" in printed
+
+    def test_unknown_command_shows_error(self):
+        """/xyz should show unknown command error."""
+        cli_obj = _make_cli()
+        cli_obj.process_command("/xyz")
+        printed = " ".join(str(c) for c in cli_obj.console.print.call_args_list)
+        assert "Unknown command" in printed
+
+    def test_exact_command_still_works(self):
+        """/help should still work as exact match."""
+        cli_obj = _make_cli()
+        with patch.object(cli_obj, 'show_help') as mock_help:
+            cli_obj.process_command("/help")
+        mock_help.assert_called_once()