feat: extension support and safer config sync

[paulbreuler]

Summary

This PR adds extension support and improves the safety of MCP client configuration syncing.

Extension System

• Extension loading: Load npm packages as extensions via config.extensions array
• Extension types: Extensions can register custom MCP tools and resources
• Per-extension config: Extension-specific configuration stored under package name keys (e.g., "@sudosandwich/limps-radix": {...})
• Lifecycle hooks: onInit and onShutdown callbacks for extensions
• Collision guards: Core tool/resource names are protected - extensions cannot override them

Safer Config Sync (sync-mcp)

• JSON Patch diff preview: Shows exactly what will change before writing to MCP client configs
• Confirmation prompt: User must confirm changes (default: no)
• Force flag: Skip confirmation with -f when scripting
• Existing config preservation: Merges changes without destroying unrelated config

Other Changes

• Fix verify-tools script to work with current exports
• Improve test temp directory usage for reliability
• Add watcher close stability improvements
• Update README with extension documentation

Test Plan

• npm run validate passes
• Extension config keys are preserved through load/save
• sync-mcp shows diff and prompts for confirmation
• sync-mcp -f skips confirmation
• Core tool/resource collision detection works

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