feat request: add Perplexity as a web search provider

[FnSK4R17s]

Add support for Perplexity as an alternative web search provider alongside Exa and Tavily.

Current state: Dexter conditionally enables web_search based on available API keys (configured in src/tools/registry.ts): Exa (EXASEARCH_API_KEY, preferred) or Tavily (TAVILY_API_KEY, fallback).

Proposed changes:

• Create a perplexitySearch tool in src/tools/search/ implementing the same web_search interface
• Add PERPLEXITY_API_KEY environment variable support
• Update the priority chain in getToolRegistry(): Exa → Perplexity → Tavily (or make it configurable)
• Update env.example with the new key

Notes: Perplexity returns grounded, citation-backed answers rather than raw search results, which could reduce the need for follow-up web_fetch calls and lower total token usage. Its API is OpenAI-compatible (chat completions with model: "sonar"), making integration straightforward.

[claytonlin1110]

@virattt please mark this as completed