Checkout-diff watch targets were per-Session, so multiple clients (phone,
desktop, browser) watching the same workspace duplicated fs.watch sets and
snapshot computation. Move all checkout-diff subscription machinery into a
singleton CheckoutDiffManager that deduplicates watchers and snapshots
across sessions — same pattern as AgentManager.
Session now keeps only a Map<subscriptionId, unsubscribe> and thin message
handler wrappers. Shared utilities (resolveCheckoutGitDir, toCheckoutError,
READ_ONLY_GIT_ENV) extracted to checkout-git-utils.ts.
Mohamed Boudra
6c3559e90f