v0.1.17 — task --result + heartbeat capture

Adds a result column on the tasks table so assignees record what they produced when closing a task.

• 5dive task done <id> --result="<text>" / 5dive task cancel <id> --result="<text>" write the column
• 5dive task ls --json and 5dive task show return it
• Heartbeat nudge updated to instruct the agent to include --result=<short summary>
• Idempotent schema migration for already-initialised stores