fix(gateway): Slack approval UX in threads — block-size overflow + typed-prefix instruction text (#43444)

Two fixes for the reported Slack thread approval UX:

1. Slack Block Kit approval/confirm sends silently overflowed the
   3000-char section-block cap (flat 2900-char truncation + header +
   reason), so long execute_code approvals failed with invalid_blocks
   and fell back to the plain-text prompt with no buttons. Budget the
   command preview against the rendered fixed parts so blocks never
   exceed the cap (send_exec_approval + send_slash_confirm).

2. The text fallbacks told users to reply /approve — which Slack blocks
   inside threads and Matrix clients reserve client-side. Add a
   typed_command_prefix capability flag on BasePlatformAdapter
   (default "/"; Slack and Matrix set "!" to match their existing
   bang-prefix rewrite) and use it in the shared fallback prompt
   builders (exec approval, update prompt, destructive slash confirm,
   expensive-model confirm) plus Matrix's reaction-prompt text.
   The slash-confirm text-intercept now also accepts bang-prefixed
   replies (!always, !cancel) since those keywords aren't registered
   commands and the adapters' rewrite doesn't touch them.

gateway/platforms/base.py

@@ -1804,6 +1804,18 @@ class BasePlatformAdapter(ABC):
     # preview (see gateway/run.py progress_callback).
     supports_code_blocks: bool = False
 
+    # The command prefix users can always TYPE on this platform to reach
+    # Hermes commands.  Default "/" (most platforms deliver "/approve" etc.
+    # as plain message text).  Platforms where typing a leading "/" is
+    # intercepted or restricted by the client (Slack blocks native slash
+    # commands inside threads; Matrix clients reserve "/" for client-local
+    # commands) ship a "!" alias rewrite in their adapter and set this to
+    # "!" so user-facing instruction text ("Reply `!approve` ...") tells
+    # users the form that actually works everywhere.  Capability flag —
+    # shared prompt builders read it via getattr(adapter,
+    # "typed_command_prefix", "/"); no per-platform branching at call sites.
+    typed_command_prefix: str = "/"
+
     def __init__(self, config: PlatformConfig, platform: Platform):
         self.config = config
         self.platform = platform