fix: Show "edit" instead of "create" in permission prompt for existing files

[charlesvien]

Problem

Write tool permission prompts always say "Create new file" even when overwriting an existing file, because the old file content was never resolved during the permission request flow.

Closes #1529

Changes

1. Add disk-read fallback in Write tool case when file isn't in the content cache
2. Pass fileContentCache and cwd to toolInfoFromToolUse in permission flow so cached content and relative paths are available

How did you test this?

Manually

[charlesvien]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• 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 👈 (View in Graphite)
• fix: Remove double scrollbar from repo selector #1537
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.