docs(plugin-history-sync): add release and migration guide

[ENvironmentSet]

Summary

• expand the plugin history sync README with setup, route, SSR, behavior, and API guidance
• document the 1.12.0 defaultHistory initial context addition
• add a 1.12-to-2.0 migration guide covering Core 3, snapshot restoration, and prevented navigation

Validation

• yarn workspace @stackflow/plugin-history-sync test --watchman=false --runInBand
• yarn workspace @stackflow/plugin-history-sync typecheck
• yarn workspace @stackflow/plugin-history-sync build
• git diff --check -- extensions/plugin-history-sync/README.md

Linear: FEP-2669

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[changeset-bot]

🦋 Changeset detected

Latest commit: 3f1f174

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

This PR includes changesets to release 0 packages

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[pkg-pr-new]

• @stackflow/demo

  yarn add https://pkg.pr.new/@stackflow/plugin-history-sync@748.tgz
  

commit: 3f1f174

[cloudflare-workers-and-pages]

Deploying stackflow-demo with    Cloudflare Pages

Latest commit:	3f1f174
Status:	✅  Deploy successful!
Preview URL:	https://19c21388.stackflow-demo.pages.dev
Branch Preview URL:	https://feature-fep-2669.stackflow-demo.pages.dev

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