Add backup exclude options and defaults

[whoiskatrin]

Summary

This PR adds end-to-end support for excluding files from backup archives.

createBackup() now supports:

• exclude: string[] for custom mksquashfs exclude patterns
• excludeDefaults: boolean to include a default ignore set

Default excludes are:
node_modules, .git, dist, build, .next, .turbo, .cache

Why

Backups currently include dependency and build directories by default,
which can make archives much larger than needed and slow down backup and
restore workflows.

API and behavior

BackupOptions.exclude is now wired through SDK -> DO -> container backup
service. Patterns are passed to mksquashfs -ef (one pattern per line).

excludeDefaults defaults to false for backward-compatible behavior.
Validation was added for exclude and excludeDefaults inputs.

Tests

Added two backup workflow E2E tests:

• restore excludes paths passed in exclude
• restore excludes default paths when excludeDefaults: true

Also ran local type checks through the pre-push hook.

Reviewer notes

This is intentionally minimal and keeps the existing backup flow. The
only functional flow change is passing an optional exclude file to
mksquashfs.

[changeset-bot]

🦋 Changeset detected

Latest commit: 313c92e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@cloudflare/sandbox	Patch

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[whoiskatrin]

Closing in favor of #437, which uses branch in this repository (no fork).