Publish the connector contract and authoring guide

Adds reference/connector-contract.md and code/connectors.md to the site, with
the node-type reference finally linking the contract it describes. Strict
build passes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

zensical.toml

@@ -43,12 +43,14 @@ nav = [
   { "Code and the CLI" = [
     { "The fluksio command" = "code/cli.md" },
     { "Writing node code" = "code/nodes.md" },
+    { "Writing a connector" = "code/connectors.md" },
     { "The HTTP API" = "code/api.md" },
     { "Remote workers" = "code/workers.md" },
     { "Agents over MCP" = "code/agents.md" },
   ] },
   { "Reference" = [
     { "Node types" = "reference/node-types.md" },
+    { "The connector contract" = "reference/connector-contract.md" },
     { "Payload types" = "reference/payload-types.md" },
     { "Configuration" = "reference/configuration.md" },
   ] },