feat: add custom HeaderPanel widget and switchable agent modes

Replace built-in Header with a custom HeaderPanel showing model name,
mode badge, and live token usage. Add AgentMode enum (normal/plan/auto)
with mode-aware permission gating — Plan mode restricts to read-only
tools, Auto mode auto-approves everything. Includes /mode slash command
and Ctrl+P keybinding to cycle modes.

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

app/models/config.py

@@ -1,6 +1,7 @@
 """Pydantic configuration models mapping to config/config.yaml."""
 
 import os
+from enum import StrEnum
 from pathlib import Path
 from typing import Any
 
@@ -8,6 +9,14 @@ import yaml
 from pydantic import BaseModel, Field, model_validator
 
 
+class AgentMode(StrEnum):
+    """Runtime agent mode controlling permission behavior."""
+
+    NORMAL = "normal"
+    PLAN = "plan"
+    AUTO = "auto"
+
+
 class LLMConfig(BaseModel):
     """LLM backend configuration."""