feat(KB-016): add task duplicate command

- Add duplicateTask() method to core store with fresh state reset
- Add CLI command handler and register duplicate subcommand
- Add POST /tasks/:id/duplicate API endpoint to dashboard
- Add comprehensive tests for store, CLI, and API routes
- Add pi extension tool for task duplication
- Add changeset for minor release bump

.changeset/add-task-duplicate.md

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+---
+"@dustinbyrne/kb": minor
+---
+
+Add `kb task duplicate` command to create a copy of any task in triage.