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/slack.toml

@@ -0,0 +1,41 @@
+id = "slack"
+name = "Slack"
+description = "Access Slack channels, messages, and users through the MCP server"
+category = "communication"
+icon = "💬"
+tags = ["chat", "messaging", "team", "channels", "collaboration"]
+
+[transport]
+type = "stdio"
+command = "npx"
+args = ["-y", "@modelcontextprotocol/server-slack"]
+
+[[required_env]]
+name = "SLACK_BOT_TOKEN"
+label = "Slack Bot Token"
+help = "A bot user OAuth token starting with xoxb-"
+is_secret = true
+get_url = "https://api.slack.com/apps"
+
+[[required_env]]
+name = "SLACK_TEAM_ID"
+label = "Slack Team ID"
+help = "Your Slack workspace team ID (found in workspace settings or URL)"
+is_secret = false
+get_url = ""
+
+[oauth]
+provider = "slack"
+scopes = ["channels:read", "chat:write", "users:read"]
+auth_url = "https://slack.com/oauth/v2/authorize"
+token_url = "https://slack.com/api/oauth.v2.access"
+
+[health_check]
+interval_secs = 60
+unhealthy_threshold = 3
+
+setup_instructions = """
+1. Go to https://api.slack.com/apps and create a new Slack app (or use an existing one). Add the 'channels:read', 'chat:write', and 'users:read' bot token scopes.
+2. Install the app to your workspace and copy the Bot User OAuth Token (starts with xoxb-).
+3. Paste the bot token and your workspace Team ID into the fields above, or use the OAuth flow to authorize directly.
+"""