feat: add web search tool with DuckDuckGo, Brave, and Google support

[novalis78]

Add a new search tool that enables agents to search the web for current information. This tool supports multiple search providers and integrates seamlessly with the existing tool system.

Features:

• DuckDuckGo search (default, no API key required)
• Brave search support
• Google search support (requires API key setup)
• Configurable max results (1-20)
• Domain filtering with site: parameter
• Rate limiting to avoid overwhelming services
• Structured results with titles, URLs, and snippets

Implementation:

• Add SearchTool to internal/agent/tools/search.go
• Register tool in coordinator.go buildTools()
• Add "search" to default allowed tools in config.go
• Add "search" to read-only tools list for task agent
• Update all relevant tests in load_test.go
• Include comprehensive documentation in search.md

The search tool is now available by default to all agents without requiring configuration, making it easy for users to query current information and documentation.

Tested with multiple providers and verified working with various query types including technical documentation searches and general information queries.

• [x] I have read CONTRIBUTING.md.
• [x] I have created a discussion that was approved by a maintainer (for new features).

[charmcli]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[novalis78]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.

[novalis78]

@andreynering quick question - did I miss another tool already capable of doing web search? I find this comes in handy so often, wondering if you got a chance to take a look.