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>

.env.example

@@ -74,6 +74,10 @@ HF_TOKEN=
 # TOOL API KEYS
 # =============================================================================
 
+# Exa API Key - AI-native web search and contents
+# Get at: https://exa.ai
+EXA_API_KEY=
+
 # Parallel API Key - AI-native web search and extract
 # Get at: https://parallel.ai
 PARALLEL_API_KEY=