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/matrix.py

@@ -422,6 +422,11 @@ class MatrixAdapter(BasePlatformAdapter):
 
     supports_code_blocks = True  # Matrix renders fenced code blocks (HTML/markdown)
 
+    # Matrix clients commonly reserve typed "/" for client-local commands;
+    # the adapter accepts "!command" as the alias that always reaches Hermes
+    # (see _normalize_matrix_bang_command), so instruction text shows "!".
+    typed_command_prefix = "!"
+
     # Threshold for detecting Matrix client-side message splits.
     # When a chunk is near the ~4000-char practical limit, a continuation
     # is almost certain.
@@ -1350,11 +1355,11 @@ class MatrixAdapter(BasePlatformAdapter):
             "⚠️ **Dangerous command requires approval**\n"
             f"```\n{cmd_preview}\n```\n"
             f"Reason: {description}\n\n"
-            "Reply `/approve` to execute, `/approve session` to approve this pattern for the session, "
-            "`/approve always` to approve permanently, or `/deny` to cancel.\n\n"
+            "Reply `!approve` to execute, `!approve session` to approve this pattern for the session, "
+            "`!approve always` to approve permanently, or `!deny` to cancel.\n\n"
             "You can also click the reaction to approve:\n"
-            "✅ = /approve\n"
-            "❎ = /deny"
+            "✅ = approve\n"
+            "❎ = deny"
         )
 
         result = await self.send(chat_id, text, metadata=metadata)