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

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