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/agent/loop.py

@@ -7,7 +7,7 @@ import time
 from typing import TYPE_CHECKING, Any
 
 from app.agent.context import SessionContext
-from app.models.config import AppConfig
+from app.models.config import AgentMode, AppConfig
 from app.models.message import Message
 from app.models.tool_call import ToolCall, ToolResult, ToolResultStatus
 from app.services.llm import LLMClient, LLMConnectionError, LLMError, LLMStreamError
@@ -59,6 +59,12 @@ class AgentLoop:
         self._skills = skills_manager
         self._skill_runner = skill_runner
         self._tools_schema = registry.get_openai_tools_schema()
+        if self._permissions.mode == AgentMode.PLAN:
+            read_only = PermissionsService.READ_ONLY_TOOLS
+            self._tools_schema = [
+                t for t in self._tools_schema
+                if t["function"]["name"] in read_only
+            ]
         self._system_prompt = self._build_system_prompt()
         self._cancelled = False
 
@@ -89,6 +95,14 @@ class AgentLoop:
                 f"\n\nCurrently active skill: {self._skill_runner.active_skill_name}. "
                 "When the skill's objective is complete, call the `finish_skill` tool."
             )
+        if self._permissions.mode == AgentMode.PLAN:
+            prompt += (
+                "\n\nYou are in PLAN mode. You may only use read-only tools: "
+                "read_file, list_dir, grep_files, find_files, finish. "
+                "Do NOT attempt to write files, edit code, or run commands. "
+                "Instead, describe what changes you would make, which files "
+                "you would modify, and provide the reasoning for each change."
+            )
         return prompt
 
     def _get_messages_with_system_prompt(self) -> list[Message]: