feat: config-gated /verbose command for messaging gateway (#3262)

* feat: config-gated /verbose command for messaging gateway

Add gateway_config_gate field to CommandDef, allowing cli_only commands
to be conditionally available in the gateway based on a config value.

- CommandDef gains gateway_config_gate: str | None — a config dotpath
  that, when truthy, overrides cli_only for gateway surfaces
- /verbose uses gateway_config_gate='display.tool_progress_command'
- Default is off (cli_only behavior preserved)
- When enabled, /verbose cycles tool_progress mode (off/new/all/verbose)
  in the gateway, saving to config.yaml — same cycle as the CLI
- Gateway helpers (help, telegram menus, slack mapping) dynamically
  check config to include/exclude config-gated commands
- GATEWAY_KNOWN_COMMANDS always includes config-gated commands so
  the gateway recognizes them and can respond appropriately
- Handles YAML 1.1 bool coercion (bare 'off' parses as False)
- 8 new tests for the config gate mechanism + gateway handler

* docs: document gateway_config_gate and /verbose messaging support

- AGENTS.md: add gateway_config_gate to CommandDef fields
- slash-commands.md: note /verbose can be enabled for messaging, update Notes
- configuration.md: add tool_progress_command to display section + usage note
- cli.md: cross-link to config docs for messaging enablement
- messaging/index.md: show tool_progress_command in config snippet
- plugins.md: add gateway_config_gate to register_command parameter table

website/docs/user-guide/cli.md

@@ -230,7 +230,7 @@ The CLI shows animated feedback as the agent works:
   ┊ 📄 web_extract (2.1s)
 ```
 
-Cycle through display modes with `/verbose`: `off → new → all → verbose`.
+Cycle through display modes with `/verbose`: `off → new → all → verbose`. This command can also be enabled for messaging platforms — see [configuration](/docs/user-guide/configuration#display-settings).
 
 ## Session Management