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/setup.py

@@ -585,11 +585,11 @@ def _print_setup_summary(config: dict, hermes_home):
     else:
         tool_status.append(("Mixture of Agents", False, "OPENROUTER_API_KEY"))
 
-    # Web tools (Parallel, Firecrawl, or Tavily)
-    if get_env_value("PARALLEL_API_KEY") or get_env_value("FIRECRAWL_API_KEY") or get_env_value("FIRECRAWL_API_URL") or get_env_value("TAVILY_API_KEY"):
+    # Web tools (Exa, Parallel, Firecrawl, or Tavily)
+    if get_env_value("EXA_API_KEY") or get_env_value("PARALLEL_API_KEY") or get_env_value("FIRECRAWL_API_KEY") or get_env_value("FIRECRAWL_API_URL") or get_env_value("TAVILY_API_KEY"):
         tool_status.append(("Web Search & Extract", True, None))
     else:
-        tool_status.append(("Web Search & Extract", False, "PARALLEL_API_KEY, FIRECRAWL_API_KEY, or TAVILY_API_KEY"))
+        tool_status.append(("Web Search & Extract", False, "EXA_API_KEY, PARALLEL_API_KEY, FIRECRAWL_API_KEY, or TAVILY_API_KEY"))
 
     # Browser tools (local Chromium or Browserbase cloud)
     import shutil