fix: Remove double scrollbar from repo selector

[charlesvien]

Problem

Repo selector dropdown shows two scrollbars when the list overflows.

Closes #1490

Changes

1. Add overflow: hidden to .rt-PopoverContent combobox reset so the outer wrapper never scrolls
2. Remove redundant inline maxHeight: "300px" from GitHubRepoPicker, deferring to the existing [cmdk-root] CSS constraint

How did you test this?

Manually

[charlesvien]

• fix: Fix stale context usage after compaction + usage calculation #1548
• fix: Show both local and remote repo name when they differ #1547
• fix: Queue messages during conversation compaction #1546
• fix: Prevent permission request from stealing focus across cells #1545
• feat: Add double-click to rename task title in session header #1544
• fix: Reduce unnecessary IPC calls causing UI unresponsiveness #1542
• fix: Move folder cleanup and orphaned worktree pruning to main process #1541
• fix: Show "edit" instead of "create" in permission prompt for existing files #1538
• fix: Remove double scrollbar from repo selector #1537 👈 (View in Graphite)
• main

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[charlesvien]

Merge activity

• Apr 8, 11:25 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Apr 8, 11:25 PM UTC: @charlesvien merged this pull request with Graphite.