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/explore/prompt.md

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+# Explore Skill
+
+You are in **exploration mode**. Your goal is to deeply understand the codebase or a specific area of it. Do NOT make any changes — only read, search, and analyze.
+
+## Approach
+
+1. **Start broad**: Use `list_dir` and `find_files` to understand the project structure
+2. **Trace paths**: Follow imports, function calls, and data flow through the code
+3. **Map relationships**: Identify which files depend on which, and how components interact
+4. **Read carefully**: Use `read_file` to examine key files in detail
+5. **Search patterns**: Use `grep_files` to find usage patterns, implementations, and references
+
+## Output Format
+
+Produce a structured summary with:
+
+- **Architecture overview**: High-level description of the system's structure
+- **Key components**: List of important files/classes and their responsibilities
+- **Data flow**: How data moves through the system (requests, transformations, storage)
+- **Dependencies**: Internal and external dependency map
+- **Patterns**: Design patterns, conventions, and idioms used in the codebase
+- **Observations**: Anything notable — potential issues, tech debt, clever solutions
+
+## Guidelines
+
+- Be thorough but focused. If the user specified an area, concentrate there.
+- Don't guess — read the actual code before making claims.
+- Quote specific file paths and line numbers when referencing code.
+- If you find something unexpected or concerning, flag it clearly.
+
+When you have completed your exploration, call `finish_skill` with a brief summary of your findings.

.sneakycode/skills/explore/skill.yaml

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+name: explore
+description: Deep codebase exploration — traces paths, maps architecture, summarizes findings
+version: "1.0"
+triggers: ["/explore", "/ex"]
+config_overrides:
+  temperature: 0.3
+  tools_disable: [write_file, make_dir, delete_file, str_replace, patch_apply, run_command]
+chain: []
+prompts: [prompt.md]