fix(forwarder): preserve checkpoint history integrity

[DedSecer]

Summary

• restore the fork-context support introduced by fix(forwarder): preserve Cursor fork context #239 while fixing the checkpoint integrity issues that caused it to be reverted
• persist the complete reachable imported Blob graph outside state.json, validate Blob IDs with SHA-256, and restore it across reloads
• publish only reference-closed checkpoint turns so Cursor never receives missing user-message or conversation-step Blob IDs
• rebuild minimal forkable turns from canonical replay for legacy conversations that already contain dangling imported turn IDs
• remove historical ThinkingMessage steps and unfinished tool calls from stable checkpoints, while retaining completed tool calls with structured results
• preserve and prune imported Blob reachability across clone, metadata sync, restart, and rewind paths

Context

PR #239 added Blob-backed checkpoint turns and fork-context restoration, but imported turn IDs were persisted without the referenced Blob contents. After a restart or later checkpoint update, Cursor could receive turn references whose user-message or step Blobs no longer existed, resulting in errors such as:

Missing user message blob id ...

The checkpoint projection also recreated historical reasoning and initial tool-call steps without a stable completed state. Cursor interpreted those restored steps as active, causing completed Thinking or Editing activity to appear as running again in later turns.

This follow-up keeps the original fork-context behavior while enforcing a closed content-addressed Blob graph and projecting only stable completed history.

Verification

• go test ./internal/backend/forwarder
• all non-root Go package tests
• all non-root Go package go vet
• git diff --check
• manual multi-turn, restart, fork, nested-fork, cancellation, and rewind testing in Cursor

Related to #260; this PR additionally covers imported Blob persistence, legacy dangling-reference recovery, rewind pruning, and stable historical step projection.

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Created with Cursor.

[leookun]

Thanks for the contribution! I’m not sure which awesome developer is behind it. Also, the previous fix for the @ conversation feature works perfectly.

At this point, I think we should be cautious about changes related to checkpoints. I previously submitted a fix for chat forking, which was actually a checkpoint fix, but the solution wasn’t ideal.

I’ll review this PR carefully. Feel free to discuss it here!

[DedSecer]

Thanks for the follow-up work on checkpoints.

After reviewing the latest main, #260, #269, and especially #284 now cover most of the original scope of this PR: Blob-backed checkpoint publication, prefetched Blob import and SHA-256 validation, Blob-only fork restoration, imported turn ID persistence, rewind-prefix handling, completed tool-call merging, and idempotent checkpoint recovery.

I’m closing this PR as largely superseded by the upstream checkpoint work.

Thanks again for incorporating and extending the fork-context handling.