move package to root level

[Soph]

so we can use entire.io/entire/gitsync

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Note

Medium Risk
Medium risk because it is a breaking change to Go import paths for any external consumers and could surface module/package path or tooling issues, though runtime logic is unchanged.

Overview
Updates the stable and unstable Go embedding APIs to live at module root (entire.io/entire/git-sync and entire.io/entire/git-sync/unstable) instead of under pkg/gitsync, and rewires imports accordingly (including internalbridge).

Refreshes README and docs (docs/architecture.md, docs/embedding.md) and adjusts CLI/benchmark/example/tests to use the new import paths.

^{Reviewed by Cursor Bugbot for commit 36f1ca9. Configure here.}