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/models/skill.py

@@ -0,0 +1,39 @@
+"""Pydantic models for structured skill packages."""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, Field
+
+
+class SkillConfigOverrides(BaseModel):
+    """Scoped config overrides applied while a skill is active."""
+
+    temperature: float | None = Field(default=None, description="Override sampling temperature")
+    max_tokens: int | None = Field(default=None, description="Override max tokens")
+    tools_enable: list[str] | None = Field(
+        default=None, description="Whitelist — only these tools available when set"
+    )
+    tools_disable: list[str] | None = Field(
+        default=None, description="Blacklist — disable specific tools"
+    )
+
+
+class SkillManifest(BaseModel):
+    """Parsed skill.yaml manifest for a skill package directory."""
+
+    name: str = Field(description="Unique skill identifier")
+    description: str = Field(description="Human-readable skill description")
+    version: str = Field(default="1.0", description="Skill version")
+    triggers: list[str] = Field(
+        default_factory=list, description="Slash commands that activate this skill"
+    )
+    config_overrides: SkillConfigOverrides = Field(
+        default_factory=SkillConfigOverrides, description="Scoped config overrides"
+    )
+    chain: list[str] = Field(
+        default_factory=list, description="Skill names to run first (dependencies)"
+    )
+    prompts: list[str] = Field(
+        default_factory=lambda: ["prompt.md"],
+        description="Markdown prompt files to load, in order",
+    )