feat(slack): register every gateway command as a native slash (Discord/Telegram parity) (#16164)

Every command in COMMAND_REGISTRY (/btw, /stop, /model, /help, /new,
/bg, /reset, ...) is now a first-class Slack slash command instead of
a /hermes <subcommand>. Users get the same autocomplete-driven slash
picker experience Slack users expect and that Discord and Telegram
already provide.

Previously Slack registered ONE native slash (/hermes) and split on
the first word, so typing /btw in Slack's composer got 'couldn't find
an app for /btw' because the workspace manifest never declared it.

Changes
- hermes_cli/commands.py: slack_native_slashes() + slack_app_manifest()
  generate a Slack manifest from the registry (canonical names +
  aliases + plugin commands), clamped to Slack's 50-slash cap with
  /hermes reserved as the catch-all.
- gateway/platforms/slack.py: single regex matcher dispatches every
  registered slash to _handle_slash_command, which dispatches on
  command['command']. Legacy /hermes <subcommand> keeps working for
  backward compat with older workspace manifests.
- hermes_cli/slack_cli.py + hermes_cli/main.py: new 'hermes slack
  manifest' command prints/writes a full manifest (display info,
  OAuth scopes, event subs, socket mode, slash commands) ready to
  paste into 'Create from manifest' or Features → App Manifest.
- hermes_cli/setup.py: _setup_slack() now writes the manifest up-front
  and points users at the 'From an app manifest' flow; also offers
  to refresh the manifest on reconfigure for picking up new commands.
- Tests: 14 new tests covering native-slash dispatch (/btw, /stop,
  /model), legacy /hermes <sub> compat, manifest structure, and
  telegram<->slack parity (every Telegram command must also register
  as a Slack slash). Existing /hermes-registration test updated to
  assert the new regex matches /hermes, /btw, /stop, /model, /help.
- Docs: slack.md gains a 'Slash Commands' section + Option A manifest
  flow in Step 1; cli-commands.md documents 'hermes slack manifest'.

Users pick up the new slashes by running 'hermes slack manifest --write'
and pasting into Features → App Manifest → Edit in their Slack app
config, then Save (Slack prompts for reinstall if scopes changed).

website/docs/reference/cli-commands.md

@@ -41,6 +41,7 @@ hermes [global-options] <command> [subcommand/options]
 | `hermes gateway` | Run or manage the messaging gateway service. |
 | `hermes setup` | Interactive setup wizard for all or part of the configuration. |
 | `hermes whatsapp` | Configure and pair the WhatsApp bridge. |
+| `hermes slack` | Slack helpers (currently: generate the app manifest with every command as a native slash). |
 | `hermes auth` | Manage credentials — add, list, remove, reset, set strategy. Handles OAuth flows for Codex/Nous/Anthropic. |
 | `hermes login` / `logout` | **Deprecated** — use `hermes auth` instead. |
 | `hermes status` | Show agent, auth, and platform status. |
@@ -221,6 +222,33 @@ hermes whatsapp
 
 Runs the WhatsApp pairing/setup flow, including mode selection and QR-code pairing.
 
+## `hermes slack`
+
+```bash
+hermes slack manifest              # print manifest to stdout
+hermes slack manifest --write      # write to ~/.hermes/slack-manifest.json
+hermes slack manifest --slashes-only  # just the features.slash_commands array
+```
+
+Generates a Slack app manifest that registers every gateway command in
+`COMMAND_REGISTRY` (`/btw`, `/stop`, `/model`, …) as a first-class
+Slack slash command — matching Discord and Telegram parity. Paste the
+output into your Slack app config at
+[https://api.slack.com/apps](https://api.slack.com/apps) → your app →
+**Features → App Manifest → Edit**, then **Save**. Slack prompts for
+reinstall if scopes or slash commands changed.
+
+| Flag | Default | Purpose |
+|------|---------|---------|
+| `--write [PATH]` | stdout | Write to a file instead of stdout. Bare `--write` writes `$HERMES_HOME/slack-manifest.json`. |
+| `--name NAME` | `Hermes` | Bot display name in Slack. |
+| `--description DESC` | default blurb | Bot description shown in the Slack app directory. |
+| `--slashes-only` | off | Emit only `features.slash_commands` for merging into a manually-maintained manifest. |
+
+Run `hermes slack manifest --write` again after `hermes update` to pick
+up any new commands.
+
+
 ## `hermes login` / `hermes logout` *(Deprecated)*
 
 :::caution