Redesign sidecar sync: baseline bootstrap + incremental patch

[michael-webster]

Summary

Replaces the current sync mechanism (merge-base recalculation + GitHub clone on every first sync) with a two-phase design:

• Bootstrap (sidecar setup or stale baseline): clones from GitHub if workspace absent, otherwise git fetch; checks out exact local HEAD when pushed or merge-base otherwise; applies a patch of all local changes; stores BaselineRef + BaselineBranch
• Incremental sync (sidecar sync, same branch, valid baseline): generates git diff <BaselineRef> --binary locally, resets sidecar to BaselineRef, applies patch — no GitHub access needed after initial setup

Staleness detection re-bootstraps automatically when the branch changes, no baseline is stored, or BaselineRef is no longer an ancestor of HEAD (rebase/force-push).

Also fixes MergeBase() fallback: was returning git rev-parse origin/HEAD (the remote tip, potentially ahead of local HEAD); now correctly uses git merge-base HEAD origin/HEAD.

Remote commands use sh -c "cd <path> && git ..." instead of git -C which is unavailable on the sidecar's older git version.

Changes

• internal/sidecar/active.go — add BaselineRef, BaselineBranch fields
• internal/gitutil/gitutil.go — add HeadRef, IsAncestor; fix MergeBase fallback
• internal/sidecar/sync.go — replace syncWorkspace/Sync with Bootstrap/Sync; remove errApplyFailed retry logic
• internal/cmd/sidecar.go — sidecarSetupSync calls Bootstrap; sync command passes cwd
• internal/sidecar/sync_test.go — updated for new behaviour
• acceptance/sidecar_test.go — sync flag test now runs from a git repo

Test plan

• All unit and acceptance tests pass (go test -race ./...)
• E2E script (dev-utils/e2e-sync.sh) passes all 4 scenarios on a live sidecar:
  • Fresh bootstrap (no baseline) → clone/fetch + patch applied
  • Incremental sync (same branch) → patch only, no re-clone
  • Branch switch → re-bootstrap triggered
  • Stale baseline (simulated rebase) → re-bootstrap triggered

🤖 Generated with Claude Code

[schurchleycci]

This seems like a good approach. A few Claude comments:

Issue 1: Growing patch size — Confirmed

In Sync (sync.go:~210), after successfully applying the patch, active.BaselineRef is never updated. The function loads the stored baseline, runs GeneratePatch(active.BaselineRef), resets, applies — then returns. Nothing writes a new BaselineRef back to state. Every subsequent call diffs from the original bootstrap SHA, so the patch grows linearly with accumulated commits. The fix would be to save active.BaselineRef = headSHA before returning.

Issue 2: Double reset in Bootstrap — Confirmed

The sequence in Bootstrap is:

git reset --hard HEAD && git clean -fd (pre-checkout)
git checkout
git reset --hard && git clean -fd (post-checkout, comment: “handles any detached HEAD drift”)
After step 2, HEAD is already at checkoutSHA in detached HEAD state. No other code runs between steps 2 and 3, so there’s nothing that could cause drift. Step 3 is an unconditional extra SSH round-trip with no effect.

Issue 3: Test coverage gaps — Confirmed

sync_test.go has exactly one test function. It covers only the stale-baseline-triggers-Bootstrap path (and specifically the sub-case where MergeBase fails). There is no test for:

The happy-path incremental sync: valid BaselineRef, same branch → patch applied, no re-bootstrap
The branch-changed trigger specifically: BaselineBranch != currentBranch → Bootstrap called
These two are the core decision paths in the new Sync logic (needsBootstrap evaluation) and aren’t exercised at all.