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

gateway/platforms/slack.py

@@ -789,24 +789,11 @@ class SlackAdapter(BasePlatformAdapter):
         user_id = command.get("user_id", "")
         channel_id = command.get("channel_id", "")
 
-        # Map subcommands to gateway commands
-        subcommand_map = {
-            "new": "/reset", "reset": "/reset",
-            "status": "/status", "stop": "/stop",
-            "help": "/help",
-            "model": "/model", "personality": "/personality",
-            "retry": "/retry", "undo": "/undo",
-            "compact": "/compress", "compress": "/compress",
-            "resume": "/resume",
-            "background": "/background",
-            "bg": "/bg",
-            "usage": "/usage",
-            "insights": "/insights",
-            "title": "/title",
-            "reasoning": "/reasoning",
-            "provider": "/provider",
-            "rollback": "/rollback",
-        }
+        # Map subcommands to gateway commands — derived from central registry.
+        # Also keep "compact" as a Slack-specific alias for /compress.
+        from hermes_cli.commands import slack_subcommand_map
+        subcommand_map = slack_subcommand_map()
+        subcommand_map["compact"] = "/compress"
         first_word = text.split()[0] if text else ""
         if first_word in subcommand_map:
             # Preserve arguments after the subcommand