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/dropbox.toml

@@ -0,0 +1,28 @@
+id = "dropbox"
+name = "Dropbox"
+description = "Access and manage Dropbox files and folders through the MCP server"
+category = "productivity"
+icon = "📦"
+tags = ["files", "storage", "cloud-storage", "sync", "sharing"]
+
+[transport]
+type = "stdio"
+command = "npx"
+args = ["-y", "@microagents/mcp-server-dropbox"]
+
+[[required_env]]
+name = "DROPBOX_ACCESS_TOKEN"
+label = "Dropbox Access Token"
+help = "A short-lived or long-lived access token from the Dropbox App Console"
+is_secret = true
+get_url = "https://www.dropbox.com/developers/apps"
+
+[health_check]
+interval_secs = 60
+unhealthy_threshold = 3
+
+setup_instructions = """
+1. Go to the Dropbox App Console (https://www.dropbox.com/developers/apps) and create a new app or select an existing one.
+2. Under the 'OAuth 2' section, generate an access token with the required permissions.
+3. Paste the access token into the DROPBOX_ACCESS_TOKEN field above.
+"""