feat: sync content definitions from core repo

Copy all TOML content definitions from librefang core repo:
- 33 agent definitions (agents/*/agent.toml)
- 14 hand definitions with docs (hands/*/HAND.toml + SKILL.md)
- 25 integration templates (integrations/*.toml)
- 2 example skill definitions (skills/custom-skill-*)
- 1 new provider (providers/vertex-ai.toml)

Part of the framework-vs-content registry split (RFC v0.7).

integrations/exa-search.toml

@@ -0,0 +1,28 @@
+id = "exa-search"
+name = "Exa Search"
+description = "Perform AI-powered neural searches and retrieve web content through the Exa MCP server"
+category = "ai"
+icon = "🔎"
+tags = ["search", "web", "ai", "neural", "semantic", "information-retrieval"]
+
+[transport]
+type = "stdio"
+command = "npx"
+args = ["-y", "exa-mcp-server"]
+
+[[required_env]]
+name = "EXA_API_KEY"
+label = "Exa API Key"
+help = "An API key from the Exa dashboard"
+is_secret = true
+get_url = "https://dashboard.exa.ai/api-keys"
+
+[health_check]
+interval_secs = 60
+unhealthy_threshold = 3
+
+setup_instructions = """
+1. Go to https://dashboard.exa.ai/ and create an account or sign in.
+2. Navigate to API Keys (https://dashboard.exa.ai/api-keys) and generate a new key.
+3. Paste the API key into the EXA_API_KEY field above.
+"""