Expose subticket relationships through the MCP tools

The MCP tools are completely blind to the parent and child structure that the board UI shows. get_ticket doesn't include the parent anywhere in its output, search_tickets has no way to filter by parent, list_links only covers relates and depends links, and the activity log doesn't record a ticket being parented or reparented. So an MCP client can't answer "what are this epic's children" or even "does this ticket have a parent" at all.

This bites in practice. An agent working an epic can only reconstruct membership from the original creation order, so tickets parented into an epic later are invisible to it, and it either misses them or claims an epic is finished when it isn't. It happened twice in one day on the JRM board.

• get_ticket includes a parentTicketRef field (null when there isn't one), in both the compact and full forms
• search_tickets accepts a parent filter that returns the direct children of the given ref
• Nice to have, the activity log records parent changes like it records status and assignee changes
• create_ticket and update_ticket can already set parentTicketRef, so this is purely about reading it back