feat(KB-034): add task archive/unarchive functionality

- Add 'archived' column to task store with archiveTask and unarchiveTask methods
- Add CLI commands: kb task archive <id> and kb task unarchive <id>
- Add pi extension tools for archive and unarchive operations
- Add dashboard API endpoints POST /api/tasks/:id/archive and /unarchive
- Add Archived column to board UI with archive/unarchive buttons
- Prevent drag-drop into archived column, add visual distinction
- Include duplicateTask from concurrent branch in merge resolution

.changeset/add-archive-tasks.md

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+---
+"@dustinbyrne/kb": patch
+---
+
+Add archive functionality for completed tasks
+
+Tasks can now be archived from the done column to keep the board focused on recent work while preserving historical tasks in an accessible but unobtrusive location.
+
+**New CLI commands:**
+- `kb task archive <id>` — Archive a done task
+- `kb task unarchive <id>` — Restore an archived task to done
+
+**Dashboard features:**
+- New "Archived" column at the end of the board (collapsed by default)
+- Archive/unarchive buttons on task cards (visible on hover)
+- Archived tasks cannot be dragged or modified
+
+**API endpoints:**
+- `POST /api/tasks/:id/archive` — Archive a task
+- `POST /api/tasks/:id/unarchive` — Unarchive a task