Add provider-native web search with Tavily fallback (#13)

* Add provider-native web search with Tavily fallback

Replace client-side Tavily web search with provider-native implementations:

- Anthropic: inject web_search_20250305 server-side tool, handle
  server_tool_use / web_search_tool_result streaming blocks, emit
  info_delta for search status display
- OpenAI: inject web_search_options for gpt-5-search-api, format
  url_citation annotations as footnote sources
- Local/vLLM: preserve existing Tavily-based web_search tool as fallback

Add supports_web_search to ModelCapabilities and info_delta to StreamChunk.
Remove end-of-life GPT-4o model entries from capability tables.
Update docs, diagrams, and README. 88 provider tests (32 new).

* Fix Copilot PR #13 review: capture streaming url_citation annotations

Accumulate url_citation annotations during OpenAI streaming and emit
formatted citations as a final info_delta chunk after the stream ends.
Previously annotations were only captured in non-streaming mode, so
search model users in the interactive path never saw citation sources.

docs/diagrams/png/03-core-engine-classes.png

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