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/linear.toml

@@ -0,0 +1,28 @@
+id = "linear"
+name = "Linear"
+description = "Manage Linear issues, projects, cycles, and teams through the MCP server"
+category = "devtools"
+icon = "📐"
+tags = ["project-management", "issues", "agile", "tracking", "sprint"]
+
+[transport]
+type = "stdio"
+command = "npx"
+args = ["-y", "linear-mcp"]
+
+[[required_env]]
+name = "LINEAR_API_KEY"
+label = "Linear API Key"
+help = "A personal API key from your Linear account settings"
+is_secret = true
+get_url = "https://linear.app/settings/api"
+
+[health_check]
+interval_secs = 60
+unhealthy_threshold = 3
+
+setup_instructions = """
+1. Open Linear and go to Settings > API (https://linear.app/settings/api).
+2. Click 'Create key' to generate a new personal API key.
+3. Paste the key into the LINEAR_API_KEY field above.
+"""