feat: add 'View full command' option to dangerous command approval (#887)

When a dangerous command is detected and the user is prompted for
approval, long commands are truncated (80 chars in fallback, 70 chars
in the TUI). Users had no way to see the full command before deciding.

This adds a 'View full command' option across all approval interfaces:

- CLI fallback (tools/approval.py): [v]iew option in the prompt menu.
  Shows the full command and re-prompts for approval decision.
- CLI TUI (cli.py): 'Show full command' choice in the arrow-key
  selection panel. Expands the command display in-place and removes
  the view option after use.
- CLI callbacks (callbacks.py): 'view' choice added to the list when
  the command exceeds 70 characters.
- Gateway (gateway/run.py): 'full', 'show', 'view' responses reveal
  the complete command while keeping the approval pending.

Includes 7 new tests covering view-then-approve, view-then-deny,
short command fallthrough, and double-view behavior.

Closes community feedback about the 80-char cap on dangerous commands.

gateway/run.py

@@ -988,6 +988,10 @@ class GatewayRunner:
             elif user_text in ("no", "n", "deny", "cancel", "nope"):
                 self._pending_approvals.pop(session_key_preview)
                 return "❌ Command denied."
+            elif user_text in ("full", "show", "view", "show full", "view full"):
+                # Show full command without consuming the approval
+                cmd = self._pending_approvals[session_key_preview]["command"]
+                return f"Full command:\n\n```\n{cmd}\n```\n\nReply yes/no to approve or deny."
             # If it's not clearly an approval/denial, fall through to normal processing
         
         # Get or create session