feat: add prompt-loader and tools-merger modules

[Gaubee]

Summary

Add core modules for the customization system: prompt loading and tools merging.

New Modules

1. prompt-loader.ts

Flexible prompt loading and rendering system.

Features:

• Multi-source priority: user > builtin
• Template syntax with configurable PREFIX/SUFFIX (default: {{KEY}})
• YAML frontmatter for custom config and ALIAS mapping
• Built-in variables: DATETIME, CWD
• Compose mode: prepend + base + append

API:

// Read prompt with priority resolution
const prompt = await readPrompt("coder");

// Render template with variables
const rendered = renderTemplate(template, { USER_RULES: rules });

// Read and render in one call
const final = await readAndRenderPrompt("coder", vars);

2. tools-merger.ts

Tool configuration merging with append mode and removal syntax.

Features:

• Append mode by default (add to existing list)
• ! prefix for removal: ["CustomTool", "!Bash"]
• Separate allow/disallow list merging

API:

// Merge tool lists
const result = mergeTools(base, ["NewTool", "!OldTool"]);

// Merge full config
const { allow, disallow } = mergeToolsConfig(
  baseAllow, 
  baseDisallow,
  { allow: ["Custom"], disallow: ["!WebSearch"] }
);

Tests

• 8 new test files with 37 test steps
• 98 total tests passing