feat: add L2 constructs (Agent, Skill, Prompt, BuiltInTool, Shell)

[alxbello]

Summary

Adds an opinionated L2 layer on top of the existing L1 constructs with builder-style APIs, sensible defaults, and composition patterns.

What's new

• Agent — wraps CfgAgent, accepts ToolConfig objects and decomposes them into tools/allowedTools/toolsSettings. Builder methods: addTool, addSkill, addPrompt, addMcpServer, addHook, addResource.
• Skill — synthesizes skills/{name}/SKILL.md with YAML frontmatter + assets. Static factory: Skill.fromDirectory().
• Prompt — wraps CfgPrompt with appendContent() and Prompt.fromFile() factory.
• BuiltInTool — typed factories for all 9 Kiro built-in tools.
• Shell — permission helpers (git.readonly(), npm.scripts(), etc.) that expand to regex patterns.
• Agent + Skill/Prompt wiring — addSkill() registers skill:// resource URI, addPrompt() registers file:// resource URI per the Kiro CLI config reference.

Tests

29 passing — tool factories, agent decomposition, skill synthesis, prompt synthesis, skill/prompt registration.