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>

.sneakycode/skills/write-document/skill.yaml

@@ -0,0 +1,8 @@
+name: write-document
+description: Draft and edit documents — READMEs, specs, changelogs, prose
+version: "1.0"
+triggers: ["/write-doc", "/doc"]
+config_overrides:
+  temperature: 0.7
+chain: []
+prompts: [prompt.md]