refactor: fix circular dependency

[ocavue]

Moves some code to fix the following "Circular dependencies" build warning. No runtime logic change.

Before:

❯ pnpm run build

src/index.ts → dist/index.js, dist/index.mjs...
(!) Circular dependencies
src/sync-plugin.ts -> src/cursor-plugin.ts -> src/sync-plugin.ts
src/sync-plugin.ts -> src/undo-plugin.ts -> src/sync-plugin.ts
created dist/index.js, dist/index.mjs in 127ms

After:

❯ pnpm run build

src/index.ts → dist/index.js, dist/index.mjs...
created dist/index.js, dist/index.mjs in 129ms

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	11.83%	612 / 5170
🔵	Statements	11.83%	612 / 5170
🔵	Functions	48.78%	20 / 41
🔵	Branches	64.74%	90 / 139

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
src/cursor-plugin.ts	0%	0%	0%	0%	1-395
src/index.ts	0%	0%	0%	0%	1-25
src/sync-plugin-key.ts	0%	0%	0%	0%	1-23
src/sync-plugin.ts	0%	0%	0%	0%	1-237
src/undo-plugin-key.ts	0%	0%	0%	0%	1-18
src/undo-plugin.ts	0%	0%	0%	0%	1-180

Generated in workflow #138 for commit 09788ae by the Vitest Coverage Report Action