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

CONTRIBUTING.md

@@ -136,7 +136,7 @@ hermes-agent/
 │   ├── auth.py                   # Provider resolution, OAuth, Nous Portal
 │   ├── models.py                 # OpenRouter model selection lists
 │   ├── banner.py                 # Welcome banner, ASCII art
-│   ├── commands.py               # Slash command definitions + autocomplete
+│   ├── commands.py               # Central slash command registry (CommandDef), autocomplete, gateway helpers
 │   ├── callbacks.py              # Interactive callbacks (clarify, sudo, approval)
 │   ├── doctor.py                 # Diagnostics
 │   ├── skills_hub.py             # Skills Hub CLI + /skills slash command