MCP: expose a task's source/connector origin link in get_task / list_tasks

Tasks created from a connector (Slack, Gmail, etc.) show an "Open in Slack/Gmail" deeplink in the Akiflow UI, but the MCP tools don't return it - get_task/list_tasks stop at links, which only holds user-attached or text-parsed URLs, never the connector source. Confirmed: get_task returns links: [] for a card with a Slack source link in the UI. This breaks AI triage over MCP - assistants rely on links to recover context, so integration-sourced tasks look "underspecified" when they actually have the richest upstream context. Affects all connectors. Please add a source/origin field (or the connector deeplink in links with a type marker) to the task payload.