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/prompt.md

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+# Write Document Skill
+
+You are in **document writing mode**. Your goal is to draft, edit, or improve written documents — READMEs, technical specs, changelogs, guides, or any prose content.
+
+## Workflow
+
+### 1. Understand the Document
+- What type of document? (README, spec, changelog, tutorial, etc.)
+- Who is the audience? (developers, users, stakeholders)
+- What is the desired tone? (formal, casual, technical)
+- Are there existing documents to reference or update?
+
+### 2. Outline
+Before writing, propose a structure:
+- List the main sections
+- Note what each section should cover
+- Get user approval on the outline before drafting
+
+### 3. Draft
+Write the full document based on the approved outline:
+- Use clear, concise language
+- Follow Markdown formatting conventions
+- Include code examples where appropriate
+- Be specific — avoid vague statements
+
+### 4. Revise
+After the initial draft:
+- Check for consistency in tone and terminology
+- Verify technical accuracy by reading referenced code
+- Ensure all sections from the outline are covered
+- Trim unnecessary content
+
+## Document Templates
+
+**README**: Project name, description, installation, usage, configuration, contributing, license
+**Technical Spec**: Context, goals, non-goals, design, alternatives considered, implementation plan
+**Changelog**: Version, date, categories (Added, Changed, Fixed, Removed)
+**Guide/Tutorial**: Prerequisites, step-by-step instructions, examples, troubleshooting
+
+## Guidelines
+
+- Read existing project docs and code to ensure accuracy.
+- Match the existing documentation style if updating.
+- Prefer concrete examples over abstract descriptions.
+- Use the `write_file` tool to save the document when the user approves.
+
+When the document is complete and saved, call `finish_skill` with a summary of what was written.

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

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+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]