refactor: centralize slash command registry (#1603)

* refactor: centralize slash command registry

Replace 7+ scattered command definition sites with a single
CommandDef registry in hermes_cli/commands.py. All downstream
consumers now derive from this registry:

- CLI process_command() resolves aliases via resolve_command()
- Gateway _known_commands uses GATEWAY_KNOWN_COMMANDS frozenset
- Gateway help text generated by gateway_help_lines()
- Telegram BotCommands generated by telegram_bot_commands()
- Slack subcommand map generated by slack_subcommand_map()

Adding a command or alias is now a one-line change to
COMMAND_REGISTRY instead of touching 6+ files.

Bugfixes included:
- Telegram now registers /rollback, /background (were missing)
- Slack now has /voice, /update, /reload-mcp (were missing)
- Gateway duplicate 'reasoning' dispatch (dead code) removed
- Gateway help text can no longer drift from CLI help

Backwards-compatible: COMMANDS and COMMANDS_BY_CATEGORY dicts are
rebuilt from the registry, so existing imports work unchanged.

* docs: update developer docs for centralized command registry

Update AGENTS.md with full 'Slash Command Registry' and 'Adding a
Slash Command' sections covering CommandDef fields, registry helpers,
and the one-line alias workflow.

Also update:
- CONTRIBUTING.md: commands.py description
- website/docs/reference/slash-commands.md: reference central registry
- docs/plans/centralize-command-registry.md: mark COMPLETED
- plans/checkpoint-rollback.md: reference new pattern
- hermes-agent-dev skill: architecture table

* chore: remove stale plan docs

tests/gateway/test_voice_command.py

@@ -475,16 +475,15 @@ class TestDiscordPlayTtsSkip:
 class TestVoiceInHelp:
 
     def test_voice_in_help_output(self):
-        from gateway.run import GatewayRunner
-        import inspect
-        source = inspect.getsource(GatewayRunner._handle_help_command)
-        assert "/voice" in source
+        """The gateway help text includes /voice (generated from registry)."""
+        from hermes_cli.commands import gateway_help_lines
+        help_text = "\n".join(gateway_help_lines())
+        assert "/voice" in help_text
 
     def test_voice_is_known_command(self):
-        from gateway.run import GatewayRunner
-        import inspect
-        source = inspect.getsource(GatewayRunner._handle_message)
-        assert '"voice"' in source
+        """The /voice command is in GATEWAY_KNOWN_COMMANDS."""
+        from hermes_cli.commands import GATEWAY_KNOWN_COMMANDS
+        assert "voice" in GATEWAY_KNOWN_COMMANDS
 
 
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