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).

hermes_cli/main.py

@@ -4780,6 +4780,37 @@ def cmd_webhook(args):
     webhook_command(args)
 
 
+def cmd_slack(args):
+    """Slack integration helpers.
+
+    Dispatches ``hermes slack <subcommand>``. Currently supports:
+      manifest — print or write a Slack app manifest with every gateway
+                 command registered as a first-class slash.
+    """
+    sub = getattr(args, "slack_command", None)
+    if sub in (None, ""):
+        # No subcommand — print usage hint.
+        print(
+            "usage: hermes slack <subcommand>\n"
+            "\n"
+            "subcommands:\n"
+            "  manifest   Generate a Slack app manifest with every gateway\n"
+            "             command registered as a native slash\n"
+            "\n"
+            "Run `hermes slack manifest -h` for details.",
+            file=sys.stderr,
+        )
+        return 1
+
+    if sub == "manifest":
+        from hermes_cli.slack_cli import slack_manifest_command
+
+        return slack_manifest_command(args)
+
+    print(f"Unknown slack subcommand: {sub}", file=sys.stderr)
+    return 1
+
+
 def cmd_hooks(args):
     """Shell-hook inspection and management."""
     from hermes_cli.hooks import hooks_command
@@ -7798,6 +7829,54 @@ For more help on a command:
     )
     whatsapp_parser.set_defaults(func=cmd_whatsapp)
 
+    # =========================================================================
+    # slack command
+    # =========================================================================
+    slack_parser = subparsers.add_parser(
+        "slack",
+        help="Slack integration helpers (manifest generation, etc.)",
+        description="Slack integration helpers for Hermes.",
+    )
+    slack_sub = slack_parser.add_subparsers(dest="slack_command")
+    slack_manifest = slack_sub.add_parser(
+        "manifest",
+        help="Print or write a Slack app manifest with every gateway command "
+             "registered as a native slash (/btw, /stop, /model, ...)",
+        description=(
+            "Generate a Slack app manifest that registers every gateway "
+            "command in COMMAND_REGISTRY as a first-class Slack slash "
+            "command (matching Discord and Telegram parity). Paste the "
+            "output into Slack app config → Features → App Manifest → "
+            "Edit, then Save. Reinstall the app if Slack prompts for it."
+        ),
+    )
+    slack_manifest.add_argument(
+        "--write",
+        nargs="?",
+        const=True,
+        default=None,
+        metavar="PATH",
+        help="Write manifest to a file instead of stdout. With no PATH "
+             "writes to $HERMES_HOME/slack-manifest.json.",
+    )
+    slack_manifest.add_argument(
+        "--name",
+        default=None,
+        help='Bot display name (default: "Hermes")',
+    )
+    slack_manifest.add_argument(
+        "--description",
+        default=None,
+        help="Bot description shown in Slack's app directory.",
+    )
+    slack_manifest.add_argument(
+        "--slashes-only",
+        action="store_true",
+        help="Emit only the features.slash_commands array (for merging "
+             "into an existing manifest manually).",
+    )
+    slack_parser.set_defaults(func=cmd_slack)
+
     # =========================================================================
     # login command
     # =========================================================================