fix(cli): handle read-only files when overwriting .preview folder

[jsklan]

Description

Fixes fern generate --preview failing with EACCES: permission denied errors when the .preview folder already exists from a previous run.

Link to Devin run: https://app.devin.ai/sessions/0807c2beab2146e980ed354374695cb8
Requested by: @jsklan

Problem

When running fern generate --preview multiple times, the CLI would fail with:

EACCES: permission denied, open '.../.preview/fern-typescript-sdk/.git/objects/...'

This happens because:

1. The generated SDK zip may include a .git directory
2. Git creates .git/objects files with read-only permissions
3. Node.js rm() fails to delete read-only files on some systems
4. The subsequent decompress fails when trying to write to existing files

Changes Made

• Added forceRemoveDirectory() helper that makes files writable before deletion
• Added makeWritableRecursive() helper to recursively chmod files to 0o755
• Updated downloadZipForTask() to use the new force removal
• Updated downloadFilesWithFernIgnoreInTempRepo() for consistency
• Added changelog entry for version 3.37.2

Testing

• Manual testing: Verified lint checks pass (pnpm run check)
• Unit tests added/updated (no new tests - the fix is in file system operations that are difficult to unit test without mocking)

Human Review Checklist

• Verify the recursive chmod approach handles edge cases safely (symlinks, special files)
• Confirm error suppression in makeWritableRecursive is appropriate (errors are ignored since files may be deleted/inaccessible mid-operation)
• Check that chmod to 0o755 is the appropriate permission level for this use case
• Note: This fix hasn't been tested on the actual failing scenario (macOS with .git/objects files) - only lint checks verified

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