Allow tasks to share a worktree

[bborn]

Summary

• Adds shared_worktree_task_id field to tasks, allowing multiple tasks to run in the same git worktree directory
• When a task has shared_worktree_task_id set, it reuses the referenced task's worktree instead of creating its own
• Cleanup/archive operations are worktree-aware: they skip removing a worktree when other active tasks still use it
• Accessible via MCP (taskyou_create_task with shared_worktree_task_id), CLI (task create --share-worktree 42), and preserved through edit/move operations

Changes

• internal/db/tasks.go: New SharedWorktreeTaskID field, CountTasksSharingWorktree helper, updated all queries/scans
• internal/db/sqlite.go: DB migration for shared_worktree_task_id column
• internal/executor/executor.go: setupWorktree handles shared worktrees; CleanupWorktree/ArchiveWorktree check shared count before removal
• internal/mcp/server.go: taskyou_create_task accepts shared_worktree_task_id parameter
• cmd/task/main.go: --share-worktree CLI flag
• internal/ui/app.go: Preserve SharedWorktreeTaskID during edit and project move

Test plan

• All existing tests pass (go test ./...)
• New tests for SharedWorktreeTaskID persistence (create, read, update)
• New tests for CountTasksSharingWorktree (active vs done tasks, multiple sharers)
• New test for MCP taskyou_create_task with shared_worktree_task_id
• Manual: Create two tasks sharing a worktree via CLI and verify they use the same directory
• Manual: Archive/cleanup one shared task and verify the worktree is preserved

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