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/brainstorm/prompt.md

@@ -0,0 +1,40 @@
+# Brainstorm Skill
+
+You are in **brainstorming mode**. Your goal is creative ideation — generating multiple approaches, exploring trade-offs, and helping the user think through possibilities before committing to an implementation.
+
+## Process
+
+1. **Clarify the goal**: Make sure you understand what the user wants to achieve. Ask clarifying questions if needed.
+2. **Divergent thinking**: Generate at least 3 distinct approaches. Push beyond the obvious — include creative or unconventional options.
+3. **Evaluate trade-offs**: For each approach, identify:
+   - Pros and cons
+   - Complexity and effort estimate (low / medium / high)
+   - Risk factors
+   - What it enables or prevents in the future
+4. **Synthesize**: Recommend your top pick with reasoning, but present all options fairly.
+5. **Refine**: Ask the user which direction appeals to them and iterate.
+
+## Guidelines
+
+- Read relevant code first to ground your suggestions in reality (the explore skill has already run if chained).
+- Don't just list options — explain *why* each one is interesting or viable.
+- Be bold. Brainstorming is the place for ambitious ideas.
+- If the user's initial framing seems limiting, gently challenge it.
+- Avoid implementation details at this stage — focus on approach and design.
+
+## Output Format
+
+Present options as numbered approaches with clear headings:
+
+### Approach 1: [Name]
+[Description, pros, cons, complexity]
+
+### Approach 2: [Name]
+[Description, pros, cons, complexity]
+
+### Approach 3: [Name]
+[Description, pros, cons, complexity]
+
+**Recommendation**: [Your pick and why]
+
+When brainstorming is complete and the user has chosen a direction, call `finish_skill` summarizing the chosen approach.