Task 02: Correct stale commits-panel base to the integration merge-base

When the resolved base commit (from the stored target branch) is a strict ancestor of the integration merge-base (origin/main → origin/master), anchor the commits panel and cumulative diff to the integration merge-base instead. Fixes the inflated 31-vs-1 commit count in the stacked-PR / merged-parent case.

Context / Inputs

Spec: docs/specs/platform/workspace-git-status.md → "Base-commit staleness and refresh", Failure modes rows, and Scenarios 1–4.
Confirmed root cause (see plan.md): stored target = merged/deleted stacked parent → computeMergeBase falls back to stale local ref (ed630b8446, 31 commits) instead of merge-base(HEAD, origin/master) (19646efc83, 1 commit).
Seam: runGitLogForRepo and computeMergeBase in apps/backend/internal/agentctl/server/api/git.go (already prefers origin/<target>).
Priority list to mirror: branchDiffCandidates in apps/backend/internal/agentctl/server/process/workspace_git_status.go (origin/main → origin/master → main → master). For the merge-base, computeMergeBase already tries origin/<name> first, so pass bare main/master.
Depends on GitOperator.IsAncestor from task 01.

Implementation

In apps/backend/internal/agentctl/server/api/git.go:

Add var integrationMergeBaseCandidates = []string{"main", "master"}.
Add func (s *Server) integrationMergeBase(ctx context.Context, gitOp *process.GitOperator) string: first non-empty computeMergeBase(HEAD, candidate) over the candidates; "" if none resolve.
Add func (s *Server) correctStaleBase(ctx, gitOp, baseCommit, targetBranch string) string:
- Return baseCommit unchanged when targetBranch is empty or is itself an integration candidate (main/master, with or without origin/ prefix) — avoid redundant self-compare.
- Compute integ := s.integrationMergeBase(...); return baseCommit if integ == "" or integ == baseCommit.
- anc, err := gitOp.IsAncestor(ctx, baseCommit, integ); if err == nil && anc && baseCommit != integ return integ (stale base is strictly behind the integration line → correct it). Otherwise return baseCommit (equal/descendant/diverged/errored → unchanged).
In runGitLogForRepo, after the existing block sets baseCommit from the target-branch merge-base (and before gitOp.GetLog), set baseCommit = s.correctStaleBase(c.Request.Context(), gitOp, baseCommit, req.TargetBranch). Preserve the existing inline safeBranchRefPattern sanitiser barriers; main/master literals are constant and safe.
Keep runGitLogForRepo within golangci limits (≤80 lines / ≤50 statements / complexity ≤15) — the logic lives in the extracted helpers above.

Area 2 (cumulative diff parity): confirm whether the live cumulative-diff handler shares this base resolution. If it computes its own base against the target branch, apply the same correctStaleBase call there. Do NOT touch lifecycleAdapter.GetCumulativeDiff (orchestrator snapshot path anchors to a caller-provided SHA by design).

Acceptance

Regression: stale stacked-parent base is replaced by merge-base(HEAD, origin/main); commits panel count equals git rev-list --first-parent --count <integ-merge-base>..HEAD (1 in repro).
A base equal to / descendant of the integration merge-base is returned unchanged.
No origin/* and unrelated history: no error, existing fallback preserved.
runGitLogForRepo passes the changed-file linter at the PR base SHA.

Verification

cd apps/backend && go test -run 'TestComputeMergeBase|TestRunGitLogForRepo|TestGetLog' ./internal/agentctl/server/api/...
cd apps/backend && golangci-lint run ./internal/agentctl/server/api/... --new-from-rev="<base-sha>" --timeout=5m

Regression tests go in apps/backend/internal/agentctl/server/api/git_log_merge_base_test.go (extend, reusing setupAPITestRepo/runGitAPI/writeFileAPI). Write the stale-parent test FIRST and confirm it fails before the code change.

Files likely touched

apps/backend/internal/agentctl/server/api/git.go
apps/backend/internal/agentctl/server/api/git_log_merge_base_test.go
(only if Area 2 requires it) apps/backend/internal/agentctl/server/process/workspace_git_status.go

Dependencies

Task 01 (GitOperator.IsAncestor).

Parallelism

sequential.

Output contract

Report: helpers added, exact insertion point in runGitLogForRepo, Area 2 finding (shared vs separate cumulative-diff base), regression test red→green evidence, go test and changed-file lint results, blockers/risks. Update this file's status and the task-02 checkbox in plan.md.

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