feat: add Exa as an optional search provider

[tgonzalezc5]

Description

Adds Exa as an optional, drop-in search provider for the search_engine tool. When EXA_API_KEY is set in the environment (or usr/.env), the agent automatically uses Exa's neural search instead of SearXNG. If the key is absent or a request fails, it falls back to SearXNG transparently.

Why Exa? Exa is an AI-native search engine built on neural embeddings. It returns semantically relevant results rather than keyword matches, which is a better fit for agent workloads where queries are natural-language questions. Highlights mode keeps token usage efficient.

Changes

• helpers/exa_search.py (new) — Thin async wrapper around the exa-py SDK. Returns results in the same {title, url, content} format the existing format_result_searxng expects, so no prompt or downstream changes are needed.
• tools/search_engine.py — Checks exa_search.is_available() at call time. Routes to Exa when configured; falls back to SearXNG on missing key or request error.
• requirements.txt — Adds exa-py>=1.0.0.

Design decisions

• Opt-in, zero breaking changes. Existing users who don't set EXA_API_KEY see no difference.
• Graceful fallback. If Exa errors mid-session, the tool retries with SearXNG rather than failing the agent loop.
• Uses models.get_api_key("exa") so it picks up API_KEY_EXA, EXA_API_KEY, or EXA_API_TOKEN automatically, consistent with how other API keys are managed in Agent Zero.
• Includes x-exa-integration: agent-zero header for integration-level usage tracking on Exa's side. (See also: HKUDS/CLI-Anything#205 for the same pattern in another open-source integration.)

Setup

# In usr/.env or your shell environment:
EXA_API_KEY=your-key-here

Get a free key at https://dashboard.exa.ai/api-keys

Test plan

• Without EXA_API_KEY: search works exactly as before (SearXNG)
• With EXA_API_KEY: search returns Exa results (title/url/highlight)
• With invalid EXA_API_KEY: gracefully falls back to SearXNG
• pip install -r requirements.txt installs exa-py without conflicts