feat(web): add Exa as a web search and extract backend (#3648)

Adds Exa (https://exa.ai) as a fourth web backend alongside Parallel,
Firecrawl, and Tavily. Follows the exact same integration pattern:

- Backend selection: config web.backend=exa or auto-detect from EXA_API_KEY
- Search: _exa_search() with highlights for result descriptions
- Extract: _exa_extract() with full text content extraction
- Lazy singleton client with x-exa-integration header
- Wired into web_search_tool and web_extract_tool dispatchers
- check_web_api_key() and requires_env updated
- CLI: hermes setup summary, hermes tools config, hermes config show
- config.py: EXA_API_KEY in OPTIONAL_ENV_VARS with metadata
- pyproject.toml: exa-py>=2.9.0,<3 in dependencies


Salvaged from PR #1850.

Co-authored-by: louiswalsh <louiswalsh@users.noreply.github.com>

pyproject.toml

@@ -26,6 +26,7 @@ dependencies = [
   # Interactive CLI (prompt_toolkit is used directly by cli.py)
   "prompt_toolkit>=3.0.52,<4",
   # Tools
+  "exa-py>=2.9.0,<3",
   "firecrawl-py>=4.16.0,<5",
   "parallel-web>=0.4.2,<1",
   "fal-client>=0.13.1,<1",