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>

hermes_cli/tools_config.py

@@ -190,6 +190,14 @@ TOOL_CATEGORIES = {
                     {"key": "FIRECRAWL_API_KEY", "prompt": "Firecrawl API key", "url": "https://firecrawl.dev"},
                 ],
             },
+            {
+                "name": "Exa",
+                "tag": "AI-native search and contents",
+                "web_backend": "exa",
+                "env_vars": [
+                    {"key": "EXA_API_KEY", "prompt": "Exa API key", "url": "https://exa.ai"},
+                ],
+            },
             {
                 "name": "Parallel",
                 "tag": "AI-native search and extract",