[Bug]: okf hub loses sync state between invocations: second push gets 409, sync reverts local edits #51

OKF Version

v0.6.0 (also reproduced on develop at 533b634)

Operating System & Architecture

macOS (Apple Silicon, arm64)

Problem Description

The sync engine keeps the last-synced head (cachedHead) and tree (cachedTree) only in memory. Every okf hub push|pull|sync runs in a new process, so each run behaves as if the device had never synced:

• Only the first push into a vault succeeds. Later pushes send no expected head and are rejected with 409 head_mismatch.
• sync reverts local edits. With an empty base tree, every locally edited file looks like a same-file collision: the remote version is restored to the canonical file and the edit is moved to <file>.conflict-local.md, even when no other device has written to the vault. Disjoint edits across devices therefore never auto-merge.
• Every file is re-encrypted and re-uploaded on each push, defeating plaintext_hash change detection (architecture/zero-knowledge-vault-sync §3.1).

No data is lost (the conflict fork keeps the edit), but hub sync is not usable for ongoing work.

The existing tests don't catch this because engine_test.go and hub_test.go reuse one in-memory Engine across steps, which the CLI never does. TestHub_Serve_E2E_Socket's conflict step also only "collides" because of this bug: device B edits a file no other device changed.

Proposed fix: persist head and tree per bundle in .okf-sync-state.json next to .okf-vault.json. Load it in HubPush/HubPull/HubSync and save it after each successful operation. Write it atomically with owner-only permissions; it is never uploaded (a dotfile, skipped by ScanBundle) and is ignored when recorded for a different vault. NewEngine stays unchanged, so library callers keep current behavior unless they opt in. I have a branch with regression tests ready and can open a PR against develop if this direction works.

Steps to Reproduce

1. okf hub serve -port 8080
2. okf hub init-vault -hub http://127.0.0.1:8080 knowledge
3. okf hub push -hub http://127.0.0.1:8080 -password pw -secret-key <key> knowledge (succeeds)
4. Edit any concept, e.g. echo edit >> knowledge/some/concept.md
5. okf hub push -hub http://127.0.0.1:8080 -password pw -secret-key <key> knowledge (fails with head conflict)
6. okf hub sync -hub http://127.0.0.1:8080 -password pw -secret-key <key> knowledge (reports a collision, reverts the edit)

Expected Behavior

A second push from the same device succeeds and re-uploads only the changed file. sync with no concurrent remote changes keeps the local edit. Disjoint edits on two devices merge without collisions.

Relevant Log Output or Terminal Screenshots

$ okf hub push ... knowledge     # first push
Push completed: commit d00bdb6f... (uploaded 30, unchanged 0)
$ echo edit >> knowledge/architecture/governance-model.md
$ okf hub push ... knowledge     # second push, same device, nobody else writing
Error: push failed: sync: head conflict, server head is "d00bdb6f..."
$ okf hub sync ... knowledge
⚠️  Collision detected at architecture/governance-model.md. Local version saved as architecture/governance-model.conflict-local.md.
Sync completed: commit 19819f9a...