fix(runtime): prompt permission mode never invokes the prompter

PermissionMode derives Ord from declaration order, placing Prompt (and Allow)
above the real privilege ladder (ReadOnly < WorkspaceWrite < DangerFullAccess).
In PermissionPolicy::authorize_with_context the ladder check
'current_mode >= required_mode' therefore evaluates true for every tool when the
active mode is Prompt (3 >= {0,1,2}), returning Allow and making the subsequent
'if current_mode == PermissionMode::Prompt' branch that routes to prompt_or_deny
unreachable.

Effect: a session configured with the 'prompt' permission mode silently
auto-allows every tool and never calls the prompter (reached via
ConversationRuntime::run_turn with an interactive prompter), defeating the mode's
entire purpose. The sibling PermissionEnforcer already special-cases Prompt
before its own 'active_mode >= required_mode' check, confirming Prompt is not
meant to be part of the ordered ladder.

Fix: exclude Prompt from the ordered-ladder comparison so Prompt mode falls
through to the interactive prompt path. Allow remains handled by its explicit
equality check. Adds regression tests covering Prompt mode with and without a
prompter.

rust/crates/runtime/src/permissions.rs

@@ -278,7 +278,7 @@ impl PermissionPolicy {
 
         if allow_rule.is_some()
             || current_mode == PermissionMode::Allow
-            || current_mode >= required_mode
+            || (current_mode != PermissionMode::Prompt && current_mode >= required_mode)
         {
             return PermissionOutcome::Allow;
         }
@@ -587,6 +587,38 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn prompt_mode_routes_to_prompter_instead_of_auto_allowing() {
+        // Regression: `Prompt` sorts above `DangerFullAccess` in the
+        // `PermissionMode` `Ord` derivation, so the `current_mode >= required_mode`
+        // ladder check used to auto-allow every tool in `Prompt` mode and never
+        // invoke the prompter — defeating the entire purpose of the mode.
+        let policy = PermissionPolicy::new(PermissionMode::Prompt)
+            .with_tool_requirement("read_file", PermissionMode::ReadOnly);
+        let mut prompter = RecordingPrompter {
+            seen: Vec::new(),
+            allow: true,
+        };
+
+        let outcome = policy.authorize("read_file", "{}", Some(&mut prompter));
+
+        assert_eq!(outcome, PermissionOutcome::Allow);
+        assert_eq!(prompter.seen.len(), 1);
+        assert_eq!(prompter.seen[0].tool_name, "read_file");
+        assert_eq!(prompter.seen[0].current_mode, PermissionMode::Prompt);
+    }
+
+    #[test]
+    fn prompt_mode_denies_when_no_prompter_is_available() {
+        let policy = PermissionPolicy::new(PermissionMode::Prompt)
+            .with_tool_requirement("read_file", PermissionMode::ReadOnly);
+
+        assert!(matches!(
+            policy.authorize("read_file", "{}", None),
+            PermissionOutcome::Deny { .. }
+        ));
+    }
+
     #[test]
     fn applies_rule_based_denials_and_allows() {
         let rules = RuntimePermissionRuleConfig::new(