[DO NOT MERGE] feat(server): fork Codex threads at checkpoints

[sameerr03]

Warning

DO NOT MERGE — personal-fork V1 experiment.

This is the backend half of the Codex edit/fork feature stack. It is based on #5 because both features coordinate destructive worktree lifecycle operations through the same server semaphore; it does not depend on the commands, selected-reply, or mobile stacks.

The problem: a fork must keep the selected conversation boundary and repository filesystem at the same point. Message order alone is not enough because Codex supports queued follow-ups and provider starts can fail before a turn exists.

How this works:

• adds a typed, operate-scoped thread-fork RPC for Codex
• resolves the exact provider-turn boundary, refusing missing, running, or non-ready checkpoints
• creates an isolated branch/worktree and restores the selected checkpoint there
• uses Codex thread/fork for native conversation context and binds it to the child thread
• records durable parent lineage without copying T3 event history
• compensates the thread, terminal, locked/partial worktree, and branch if creation, setup, or native forking fails

Validation:

• server and contracts typechecks
• 82 projection, provider-service, and Codex-adapter tests
• 49 authorization and orchestration contract tests
• focused WebSocket fork/boundary/cleanup tests
• real Git test proving forced cleanup removes a locked worktree
• real T3 web-client run proving an assistant fork retained native context and left the parent unchanged
• targeted lint and formatting checks
• adversarial review: no remaining correctness or data-loss findings

The real-client pass caught a Git timeout that occurred after a worktree was partially created. Follow-up commit 38fd5b0e7 precomputes the exact worktree path and compensates even when create returns an error; the reproduction artifact was removed.

Stack:

• prerequisite: [DO NOT MERGE] feat(server): remove worktrees when archiving #5 archive lifecycle coordination
• next: [DO NOT MERGE] feat(web): edit and fork Codex conversations #8 web edit/fork actions and lineage UI

Built with GPT-5.6-Sol through the Codex harness in T3 Code.

[github-actions]

Thread transfer impact

⚠️ The latest CI run did not produce a thread transfer result for 38fd5b0.

This comment will update automatically after the next completed run.