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/config.py

@@ -623,6 +623,14 @@ OPTIONAL_ENV_VARS = {
     },
 
     # ── Tool API keys ──
+    "EXA_API_KEY": {
+        "description": "Exa API key for AI-native web search and contents",
+        "prompt": "Exa API key",
+        "url": "https://exa.ai/",
+        "tools": ["web_search", "web_extract"],
+        "password": True,
+        "category": "tool",
+    },
     "PARALLEL_API_KEY": {
         "description": "Parallel API key for AI-native web search and extract",
         "prompt": "Parallel API key",
@@ -1679,6 +1687,7 @@ def show_config():
     keys = [
         ("OPENROUTER_API_KEY", "OpenRouter"),
         ("VOICE_TOOLS_OPENAI_KEY", "OpenAI (STT/TTS)"),
+        ("EXA_API_KEY", "Exa"),
         ("PARALLEL_API_KEY", "Parallel"),
         ("FIRECRAWL_API_KEY", "Firecrawl"),
         ("TAVILY_API_KEY", "Tavily"),
@@ -1838,7 +1847,7 @@ def set_config_value(key: str, value: str):
     # Check if it's an API key (goes to .env)
     api_keys = [
         'OPENROUTER_API_KEY', 'OPENAI_API_KEY', 'ANTHROPIC_API_KEY', 'VOICE_TOOLS_OPENAI_KEY',
-        'PARALLEL_API_KEY', 'FIRECRAWL_API_KEY', 'FIRECRAWL_API_URL', 'TAVILY_API_KEY',
+        'EXA_API_KEY', 'PARALLEL_API_KEY', 'FIRECRAWL_API_KEY', 'FIRECRAWL_API_URL', 'TAVILY_API_KEY',
         'BROWSERBASE_API_KEY', 'BROWSERBASE_PROJECT_ID', 'BROWSER_USE_API_KEY',
         'FAL_KEY', 'TELEGRAM_BOT_TOKEN', 'DISCORD_BOT_TOKEN',
         'TERMINAL_SSH_HOST', 'TERMINAL_SSH_USER', 'TERMINAL_SSH_KEY',