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/notion.toml

@@ -0,0 +1,28 @@
+id = "notion"
+name = "Notion"
+description = "Access and manage Notion pages, databases, and blocks through the MCP server"
+category = "productivity"
+icon = "📝"
+tags = ["notes", "wiki", "knowledge-base", "documentation", "databases"]
+
+[transport]
+type = "stdio"
+command = "npx"
+args = ["-y", "@notionhq/notion-mcp-server"]
+
+[[required_env]]
+name = "NOTION_API_KEY"
+label = "Notion Integration Token"
+help = "An internal integration token created in your Notion workspace settings"
+is_secret = true
+get_url = "https://www.notion.so/my-integrations"
+
+[health_check]
+interval_secs = 60
+unhealthy_threshold = 3
+
+setup_instructions = """
+1. Go to https://www.notion.so/my-integrations and click 'New integration'.
+2. Give it a name, select your workspace, and grant the required capabilities (Read/Update/Insert content).
+3. Copy the Internal Integration Token and paste it into the NOTION_API_KEY field above. Then share relevant pages with the integration in Notion.
+"""