fix: Persist new task draft across navigation

[charlesvien]

Problem

Task input draft in command center cells is lost when navigating away because the creating state uses local useState that resets on unmount.

Changes

1. Move isCreating state to persisted creatingCells in command center store
2. Give each cell a unique draft session ID (cc-cell-{index}) to avoid collisions
3. Accept optional sessionId prop on TaskInput for cell-specific drafts
4. Clean up creating state and drafts on task assignment, cancel and layout resize

How did you test this?

Manually

[charlesvien]

• feat: Replace preview session process spawn with lightweight config query #1438
• fix: Fix model selection lost during task creation #1437
• fix: Persist new task draft across navigation #1436 👈 (View in Graphite)
• main

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