feat: Add prompt history to task input and fix command center text selection

[charlesvien]

Problem

Task input page has no prompt history navigation, and selecting text in the command center loses its highlight on selection.

Changes

1. Add persisted task input history store (last 50 prompts, deduped)
2. Generalize prompt history store to accept history arrays instead of task IDs
3. Wire up/down arrow navigation on the task input page via getPromptHistory
4. Update placeholder to mention ↑↓ for history
5. Fix command center click handler to skip focus steal when text is selected

How did you test this?

Manually

[charlesvien]

• fix: Remove snapshot APIs that deadlock libuv thread pool #1338
• fix: Detect non-standard default branch names #1336
• feat: Add prompt history to task input and fix command center text selection #1335 👈 (View in Graphite)
• fix: Only send 1M context beta header for supported models #1334
• main

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[charlesvien]

Merge activity

• Mar 30, 2:57 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Mar 30, 3:00 PM UTC: Graphite rebased this pull request as part of a merge.
• Mar 30, 3:07 PM UTC: @charlesvien merged this pull request with Graphite.