feat(sync-backend): add renameFolderPrefix mutation

[github-actions]

Description

Add a renameFolderPrefix mutation to the sync backend that atomically renames all file and asset paths under a given folder prefix.

Closes #47

Behavior

• Normalizes prefixes to ensure trailing-slash folder boundaries (prevents docs/old matching docs/older).
• Preflights target-path collisions and throws a deterministic error listing conflicting paths.
• Atomically patches all matching file and asset paths in a single Convex transaction.
• Returns { renamedFiles, renamedAssets } counts.
• No-ops when old/new prefixes are equal or no files match.

Out of scope (follow-ups)

• UI integration (sidebar rename, compact-row semantics).
• Cross-prefix markdown link/asset reference rewriting.
• Sidebar expanded-state key migration.
• Empty-folder preservation.

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

• Existing tests pass
• Added new tests for changes
• Tested manually (describe below)

Manual Testing Details:

• pnpm check (biome lint/format) passes with no errors.
• tsc --noEmit against packages/sync-backend/convex/tsconfig.json passes.

Checklist

• I discussed this change in a GitHub issue before submitting this PR
• I have run the linter, formatter, and tests to ensure my code is ready for review

Co-Authored-By: Oz oz-agent@warp.dev