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/services/permissions.py

@@ -8,7 +8,7 @@ import re
 import shlex
 from collections.abc import Awaitable, Callable
 
-from app.models.config import PermissionsConfig, ToolsConfig
+from app.models.config import AgentMode, PermissionsConfig, ToolsConfig
 
 logger = logging.getLogger(__name__)
 
@@ -30,6 +30,10 @@ class PermissionsService:
     shows a modal dialog. Without a callback, unlisted tools are denied.
     """
 
+    READ_ONLY_TOOLS: frozenset[str] = frozenset({
+        "read_file", "list_dir", "grep_files", "find_files", "finish",
+    })
+
     def __init__(
         self,
         config: PermissionsConfig,
@@ -38,6 +42,16 @@ class PermissionsService:
         self.config = config
         self._tools_config = tools_config
         self._prompt_callback: PromptCallback | None = None
+        self._mode: AgentMode = AgentMode.NORMAL
+
+    @property
+    def mode(self) -> AgentMode:
+        """Current agent mode."""
+        return self._mode
+
+    @mode.setter
+    def mode(self, value: AgentMode) -> None:
+        self._mode = value
 
     def set_prompt_callback(self, callback: PromptCallback) -> None:
         """Set the async callback used to prompt the user for permission.
@@ -67,6 +81,16 @@ class PermissionsService:
             logger.info("Tool '%s' is in deny list — blocked", tool_name)
             return False
 
+        if self._mode == AgentMode.AUTO:
+            logger.debug("Tool '%s' auto-approved (AUTO mode)", tool_name)
+            return True
+
+        if self._mode == AgentMode.PLAN:
+            if tool_name not in self.READ_ONLY_TOOLS:
+                logger.info("Tool '%s' blocked in Plan mode (read-only tools only)", tool_name)
+                return False
+            return True
+
         if tool_name in self.config.auto_approve:
             logger.debug("Tool '%s' is auto-approved", tool_name)
             return True