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).

tests/gateway/test_slack.py

@@ -147,7 +147,20 @@ class TestAppMentionHandler:
         assert "app_mention" in registered_events
         assert "assistant_thread_started" in registered_events
         assert "assistant_thread_context_changed" in registered_events
-        assert "/hermes" in registered_commands
+        # Slack slash commands are registered via a single regex matcher
+        # covering every COMMAND_REGISTRY entry (e.g. /hermes, /btw, /stop,
+        # /model, ...) so users get native-slash parity with Discord and
+        # Telegram. Verify the regex matches the key expected slashes.
+        assert len(registered_commands) == 1, (
+            f"expected 1 combined slash matcher, got {registered_commands!r}"
+        )
+        slash_matcher = registered_commands[0]
+        import re as _re
+        assert isinstance(slash_matcher, _re.Pattern)
+        for expected in ("/hermes", "/btw", "/stop", "/model", "/help"):
+            assert slash_matcher.match(expected), (
+                f"Slack slash regex does not match {expected}"
+            )
 
 
 class TestSlackConnectCleanup:
@@ -1544,6 +1557,83 @@ class TestSlashCommands:
         msg = adapter.handle_message.call_args[0][0]
         assert msg.text == "/reasoning"
 
+    # ------------------------------------------------------------------
+    # Native slash commands — /btw, /stop, /model, ... dispatched directly
+    # instead of as /hermes subcommands. This is the Discord/Telegram parity
+    # fix: the slash name itself becomes the command.
+    # ------------------------------------------------------------------
+
+    @pytest.mark.asyncio
+    async def test_native_btw_slash(self, adapter):
+        """/btw with args must dispatch to /background, not /hermes btw."""
+        command = {
+            "command": "/btw",
+            "text": "fix the failing test",
+            "user_id": "U1",
+            "channel_id": "C1",
+        }
+        await adapter._handle_slash_command(command)
+        msg = adapter.handle_message.call_args[0][0]
+        # The gateway command dispatcher resolves /btw -> background via
+        # resolve_command() — our handler's job is just to deliver
+        # "/btw <args>" to the gateway runner, which is what this asserts.
+        assert msg.text == "/btw fix the failing test"
+
+    @pytest.mark.asyncio
+    async def test_native_stop_slash_no_args(self, adapter):
+        command = {
+            "command": "/stop",
+            "text": "",
+            "user_id": "U1",
+            "channel_id": "C1",
+        }
+        await adapter._handle_slash_command(command)
+        msg = adapter.handle_message.call_args[0][0]
+        assert msg.text == "/stop"
+
+    @pytest.mark.asyncio
+    async def test_native_model_slash_with_args(self, adapter):
+        command = {
+            "command": "/model",
+            "text": "anthropic/claude-sonnet-4",
+            "user_id": "U1",
+            "channel_id": "C1",
+        }
+        await adapter._handle_slash_command(command)
+        msg = adapter.handle_message.call_args[0][0]
+        assert msg.text == "/model anthropic/claude-sonnet-4"
+
+    @pytest.mark.asyncio
+    async def test_legacy_hermes_prefix_still_works(self, adapter):
+        """Backward compat: /hermes btw foo must still route to /btw foo.
+
+        Old workspace manifests only declared /hermes as the single slash.
+        After users refresh their manifest they get /btw natively, but the
+        legacy form must keep working during the transition.
+        """
+        command = {
+            "command": "/hermes",
+            "text": "btw run the tests",
+            "user_id": "U1",
+            "channel_id": "C1",
+        }
+        await adapter._handle_slash_command(command)
+        msg = adapter.handle_message.call_args[0][0]
+        assert msg.text == "/btw run the tests"
+
+    @pytest.mark.asyncio
+    async def test_legacy_hermes_freeform_question(self, adapter):
+        """/hermes <free-form text> must stay as the raw text (non-command)."""
+        command = {
+            "command": "/hermes",
+            "text": "what's the weather today?",
+            "user_id": "U1",
+            "channel_id": "C1",
+        }
+        await adapter._handle_slash_command(command)
+        msg = adapter.handle_message.call_args[0][0]
+        assert msg.text == "what's the weather today?"
+
 
 # ---------------------------------------------------------------------------
 # TestMessageSplitting