feat: add Tavily search support with Brave-compatible config

[akirose]

Summary

This PR expands web_search from Brave-only to a multi-provider setup by adding first-class Tavily support.
It centralizes search configuration through WebSearchConfig and wires the same config through both the main agent and subagents so they share identical search behavior.

It also preserves backward compatibility for legacy Brave config (tools.web.search.apiKey) and adds stronger validation/normalization plus regression tests.

Changed files

File	Change
.gitignore	Removed tests/ ignore rule
README.md	Added Tavily setup examples, option docs, and compatibility notes
nanobot/config/schema.py	Extended WebSearchConfig (provider, brave_api_key, tavily_api_key, tavily_search_depth) with normalization helpers
nanobot/agent/tools/web.py	Refactored WebSearchTool to support both Brave and Tavily, with provider-specific request/error handling
nanobot/agent/loop.py	Added web_search_config injection and passed it to tool setup + SubagentManager
nanobot/agent/subagent.py	Added web_search_config support so subagents use the same search settings
nanobot/cli/commands.py	Passed full web_search_config into both AgentLoop construction paths
tests/test_web_search.py	Added end-to-end behavior tests for Brave/Tavily success and failure cases
tests/test_web_search_tool.py	Added provider key forwarding and Tavily depth validation tests
tests/test_web_search_config.py	Added schema normalization/validation and legacy key fallback tests
tests/test_search_injection_path.py	Verified agent/subagent share the same injected search config
tests/test_config_search_schema.py	Verified camelCase config key conversion and schema mapping

Config example

{
  "tools": {
    "web": {
      "search": {
        "provider": "tavily",
        "tavilyApiKey": "tvly-xxx",
        "maxResults": 5,
        "tavilySearchDepth": "advanced"
      }
    }
  }
}

Brave example:

{
  "tools": {
    "web": {
      "search": {
        "provider": "brave",
        "braveApiKey": "BSA-xxx",
        "maxResults": 5
      }
    }
  }
}

Test plan

• Verify WebSearchConfig normalizes provider and tavilySearchDepth (trim + lowercase)
• Verify invalid provider / tavilySearchDepth raises validation errors
• Verify Brave mode uses braveApiKey and still supports legacy apiKey fallback
• Verify Tavily mode forwards tavilyApiKey, search_depth, and max_results correctly
• Verify config propagation path AgentLoop -> SubagentManager -> WebSearchTool shares the same config object
• Verify timeout/missing-key/unsupported-provider paths return clear user-facing errors

[Athemis]

There's another PR which implements support for multiple search engines including Tavily #398

[chengyongru]

Because this issue has been inactive for a long time, I will close it. If there are any other problems, please feel free to open a new issue.