opentui(phase3): launcher integration — HERMES_TUI_ENGINE dual-engine

hermes --tui launches the native OpenTUI engine (Bun) when
HERMES_TUI_ENGINE=opentui (env) or display.tui_engine=opentui (config);
Ink stays the default and the shipping path is untouched.

- _resolve_tui_engine() (env > config > ink); refuses opentui on
  Windows/Termux (no Bun) -> falls back to ink with a notice.
- _make_opentui_argv() -> [bun, src/entry.real.tsx] (no build step).
- _bun_bin() with HERMES_BUN override.
- Branch at top of _make_tui_argv BEFORE _ensure_tui_node (Bun-only host
  must not bootstrap Node).
- Gate _launch_tui NODE_OPTIONS/--max-old-space-size on engine==ink (Bun
  is JSC; the V8 flag errors/ignores).

Verified end-to-end via tmux: real hermes --tui -> Bun -> OpenTUI ->
real Python gateway streamed a real reply. No-flag default still ink.

gateway/platforms/slack.py

@@ -318,11 +318,6 @@ class SlackAdapter(BasePlatformAdapter):
 
     MAX_MESSAGE_LENGTH = 39000  # Slack API allows 40,000 chars; leave margin
     supports_code_blocks = True  # Slack mrkdwn renders fenced code blocks
-    # Slack blocks typed native slash commands inside threads ("/approve is
-    # not supported in threads. Sorry!").  The adapter rewrites a leading
-    # "!" to "/" for known commands (see _handle_slack_message), so "!" is
-    # the prefix that works everywhere — instruction text must show it.
-    typed_command_prefix = "!"
 
     def __init__(self, config: PlatformConfig):
         super().__init__(config, Platform.SLACK)
@@ -2697,26 +2692,19 @@ class SlackAdapter(BasePlatformAdapter):
             return SendResult(success=False, error="Not connected")
 
         try:
+            cmd_preview = command[:2900] + "..." if len(command) > 2900 else command
             thread_ts = self._resolve_thread_ts(None, metadata)
 
-            # Slack hard-caps a section block's text at 3000 chars; an
-            # oversized block fails the whole send with ``invalid_blocks``
-            # and the gateway falls back to the plain-text prompt (no
-            # buttons).  execute_code approvals embed the entire script in
-            # ``command``, so budget the preview against the fixed parts
-            # instead of a flat truncation that overflows once the header +
-            # reason are added.
-            header = ":warning: *Command Approval Required*\n"
-            reason = f"Reason: {description[:500]}"
-            budget = 3000 - len(header) - len(reason) - len("``````\n") - len("...")
-            cmd_preview = command[:budget] + "..." if len(command) > budget else command
-
             blocks = [
                 {
                     "type": "section",
                     "text": {
                         "type": "mrkdwn",
-                        "text": f"{header}```{cmd_preview}```\n{reason}",
+                        "text": (
+                            f":warning: *Command Approval Required*\n"
+                            f"```{cmd_preview}```\n"
+                            f"Reason: {description}"
+                        ),
                     },
                 },
                 {
@@ -2784,13 +2772,8 @@ class SlackAdapter(BasePlatformAdapter):
             return SendResult(success=False, error="Not connected")
 
         try:
+            body = message[:2900] + "..." if len(message) > 2900 else message
             thread_ts = self._resolve_thread_ts(None, metadata)
-            # Same 3000-char section-block cap as send_exec_approval: budget
-            # the body against the rendered title so the wrapper never pushes
-            # the block over the limit (overflow → invalid_blocks → no buttons).
-            _title = (title or "Confirm")[:150]
-            budget = 3000 - len(f"*{_title}*\n\n") - len("...")
-            body = message[:budget] + "..." if len(message) > budget else message
             # Encode session_key and confirm_id into the button value so the
             # callback handler can resolve without extra bookkeeping.
             value = f"{session_key}|{confirm_id}"
@@ -2800,7 +2783,7 @@ class SlackAdapter(BasePlatformAdapter):
                     "type": "section",
                     "text": {
                         "type": "mrkdwn",
-                        "text": f"*{_title}*\n\n{body}",
+                        "text": f"*{title or 'Confirm'}*\n\n{body}",
                     },
                 },
                 {