Derive parent task status from subtasks and confirm group updates

[arkalon76]

Parent tasks previously kept an independent status, so a parent could appear done while its subtasks were unfinished. Parent status now derives from descendant tasks, including tasks nested under bullets: all Todo stays Todo, all Done becomes Done, and mixed or active work becomes In Progress.

Status shortcuts on a parent now show a warning before updating the entire task group. Confirmation creates one undoable change; cancellation leaves tasks and undo history untouched. The derived status is used consistently in tree and agenda rendering, the dashboard, CLI output, persistence, and completed-task filtering and archiving. README and wiki guidance describe the new behavior.

Validation: go test ./..., go build -o halptask ., and git diff --check passed. Regression tests cover nested aggregation, persistence, guarding direct parent edits, cancellation, confirmation, undo/redo, and preserving incomplete subtrees during completed-task cleanup.

Closes #8.