[P0.3] Worktree isolation must not silently discard external-engine work (`merge_back` is dead code)

[frankbria]

Severity: critical | Category: data-integrity | Phase: P0.3

Problem

Verified: grep merge_back shows only the definition + docstring — zero call sites. External engines write to cf/<task_id> in the worktree, then cleanup() runs git worktree remove --force + git branch -D in a finally, destroying all changes; the task is marked COMPLETED with no surviving code.

Evidence

codeframe/core/worktrees.py:80 (defined), codeframe/core/sandbox/context.py:81 + codeframe/core/runtime.py:931 (cleanup force-deletes)

Acceptance criteria

• On successful worktree run, merge_back() (or a patch export) runs before cleanup; merge conflicts surface as blockers and the branch is preserved on failure.
• Integration test: a file created by the adapter under --isolation worktree exists on the base branch after the run.
• (Acceptable interim: hard-reject --isolation worktree with a clear error until merge-back ships, so no work is lost.)

Dependencies

None

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_{Filed from the SaaS launch-readiness audit. Atomic: one developer, one session. Work order: strictly P0.1 → P3.12 (no forward dependencies).}

[coderabbitai]

🔗 Related PRs

#436 - feat(adapters): detect modified files via git diff after subprocess execution [merged]
#439 - feat(core): continuous reconciliation during batch execution [merged]
#441 - feat(core): worktree-per-task isolation for parallel batch execution [merged]
#537 - feat(core): WorktreeRegistry, orphan cleanup, and get_base_branch (#533) [merged]
#541 - feat(core): absorb parallel-cc worktree + E2B into CodeFrame namespace (#535) [merged]

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