feat: structured skill packages with config overrides, chaining, and TUI integration

Add a skill package system where each skill is a directory with a skill.yaml
manifest and prompt markdown files. Skills support /command triggers, scoped
config overrides (temperature, max_tokens, tool filtering), chain dependencies
with cycle-safe resolution, and a finish_skill completion signal.

Includes four built-in skills: explore, brainstorm, write-document, and plan.

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

app/ui/widgets.py

@@ -147,6 +147,7 @@ class StatusBar(Static):
         self._spinner_frame: int = 0
         self._spinner_timer: Timer | None = None
         self._stream_tokens: int = 0
+        self._active_skill: str | None = None
 
     def update_tokens(self, tokens: int, budget: int) -> None:
         """Update the token usage display."""
@@ -184,9 +185,16 @@ class StatusBar(Static):
         self._spinner_frame = (self._spinner_frame + 1) % len(self._SPINNER)
         self._refresh_display()
 
+    def set_active_skill(self, skill_name: str | None) -> None:
+        """Set or clear the active skill indicator."""
+        self._active_skill = skill_name
+        self._refresh_display()
+
     def _refresh_display(self) -> None:
         """Rebuild the status bar text."""
         parts: list[str] = []
+        if self._active_skill:
+            parts.append(f"[Skill: {self._active_skill}]")
         if self._streaming:
             spinner = self._SPINNER[self._spinner_frame]
             parts.append(f"{spinner} Thinking")