feat: implement tweaks plan - modals, smart shell, spinner, /models, debug log, skills

Phase 1: Permission modal dialog, session resume modal, HistoryInput with
up/down arrow cycling, remove "You:" echo from chat log, LLM client cleanup
on unmount.

Phase 2: Smart shell auto-approve using allowed/denied command lists from
ToolsConfig, animated thinking spinner with live token count in status bar.

Phase 3: /models slash command (list + switch), CLI directory positional
argument, JSONL debug logger with rotation.

Phase 4: Skills system with SkillsManager, load_skill LLM tool, /skills
listing, skill invocation via slash commands, system prompt integration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/ui/styles.tcss

@@ -21,6 +21,35 @@ Screen {
     display: block;
 }
 
+/* Modal dialog */
+#permission-dialog {
+    width: 60;
+    height: auto;
+    max-height: 12;
+    border: thick $accent;
+    background: $surface;
+    padding: 1 2;
+}
+
+.modal-title {
+    text-style: bold;
+    margin-bottom: 1;
+}
+
+.modal-body {
+    margin-bottom: 1;
+    color: $text-muted;
+}
+
+.modal-buttons {
+    height: 3;
+    align: center middle;
+}
+
+.modal-buttons Button {
+    margin: 0 1;
+}
+
 /* StatusBar styles are in DEFAULT_CSS on the widget itself */
 
 Input {