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

AGENTS.md

@@ -129,14 +129,50 @@ Messages follow OpenAI format: `{"role": "system/user/assistant/tool", ...}`. Re
 - **KawaiiSpinner** (`agent/display.py`) — animated faces during API calls, `┊` activity feed for tool results
 - `load_cli_config()` in cli.py merges hardcoded defaults + user config YAML
 - **Skin engine** (`hermes_cli/skin_engine.py`) — data-driven CLI theming; initialized from `display.skin` config key at startup; skins customize banner colors, spinner faces/verbs/wings, tool prefix, response box, branding text
-- `process_command()` is a method on `HermesCLI` (not in commands.py)
+- `process_command()` is a method on `HermesCLI` — dispatches on canonical command name resolved via `resolve_command()` from the central registry
 - Skill slash commands: `agent/skill_commands.py` scans `~/.hermes/skills/`, injects as **user message** (not system prompt) to preserve prompt caching
 
-### Adding CLI Commands
+### Slash Command Registry (`hermes_cli/commands.py`)
 
-1. Add to `COMMANDS` dict in `hermes_cli/commands.py`
-2. Add handler in `HermesCLI.process_command()` in `cli.py`
-3. For persistent settings, use `save_config_value()` in `cli.py`
+All slash commands are defined in a central `COMMAND_REGISTRY` list of `CommandDef` objects. Every downstream consumer derives from this registry automatically:
+
+- **CLI** — `process_command()` resolves aliases via `resolve_command()`, dispatches on canonical name
+- **Gateway** — `GATEWAY_KNOWN_COMMANDS` frozenset for hook emission, `resolve_command()` for dispatch
+- **Gateway help** — `gateway_help_lines()` generates `/help` output
+- **Telegram** — `telegram_bot_commands()` generates the BotCommand menu
+- **Slack** — `slack_subcommand_map()` generates `/hermes` subcommand routing
+- **Autocomplete** — `COMMANDS` flat dict feeds `SlashCommandCompleter`
+- **CLI help** — `COMMANDS_BY_CATEGORY` dict feeds `show_help()`
+
+### Adding a Slash Command
+
+1. Add a `CommandDef` entry to `COMMAND_REGISTRY` in `hermes_cli/commands.py`:
+```python
+CommandDef("mycommand", "Description of what it does", "Session",
+           aliases=("mc",), args_hint="[arg]"),
+```
+2. Add handler in `HermesCLI.process_command()` in `cli.py`:
+```python
+elif canonical == "mycommand":
+    self._handle_mycommand(cmd_original)
+```
+3. If the command is available in the gateway, add a handler in `gateway/run.py`:
+```python
+if canonical == "mycommand":
+    return await self._handle_mycommand(event)
+```
+4. For persistent settings, use `save_config_value()` in `cli.py`
+
+**CommandDef fields:**
+- `name` — canonical name without slash (e.g. `"background"`)
+- `description` — human-readable description
+- `category` — one of `"Session"`, `"Configuration"`, `"Tools & Skills"`, `"Info"`, `"Exit"`
+- `aliases` — tuple of alternative names (e.g. `("bg",)`)
+- `args_hint` — argument placeholder shown in help (e.g. `"<prompt>"`, `"[name]"`)
+- `cli_only` — only available in the interactive CLI
+- `gateway_only` — only available in messaging platforms
+
+**Adding an alias** requires only adding it to the `aliases` tuple on the existing `CommandDef`. No other file changes needed — dispatch, help text, Telegram menu, Slack mapping, and autocomplete all update automatically.
 
 ---