fix: use full DAG walk for explain on default branch

[khaong]

Summary

Follow-up to #203 — fixes a regression where entire explain on the default branch (main/master) only showed checkpoints from first-parent commits, missing checkpoint commits that came in via merged feature branches (second parents of merge commits).

• On the default branch: use repo.Log() full DAG walk (no branch filtering needed), finding checkpoints on all reachable commits including merged feature branches
• On feature branches: use walkFirstParentCommits with reachableFromMain filtering (unchanged from fix: use first-parent traversal in explain to handle merge commits #203)
• Extract collectCheckpoint helper to avoid duplicating the checkpoint-to-RewindPoint conversion between the two code paths

Context

The original repo.Log() code worked on both default and feature branches because on the default branch, reachableFromMain was empty so no filtering happened. #203 replaced repo.Log() with walkFirstParentCommits everywhere, which fixed feature branches but broke the default branch by only walking the first-parent chain.

Test plan

• TestGetBranchCheckpoints_DefaultBranchFindsMergedCheckpoints — creates merge commit on master with checkpoint-tagged feature branch commit as second parent, verifies it's found
• All existing explain tests pass
• Integration tests pass with -race

🤖 Generated with Claude Code

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Note

Low Risk
Behavior change is limited to how getBranchCheckpoints enumerates commits on the default branch, with added test coverage for the merge-parent case; minimal impact outside entire explain output.

Overview
Fixes a regression in entire explain branch view where, on the default branch, checkpoint commits that arrived via merge commits (i.e., on non-first-parent paths) were not shown.

getBranchCheckpoints now uses a full repo.Log() DAG walk (with a scan limit) when on the default branch, while keeping the existing first-parent traversal plus reachableFromMain filtering on feature branches; the checkpoint-to-RewindPoint conversion is factored into a shared collectCheckpoint helper. Adds a regression test that constructs a real merge commit on master and asserts merged feature-branch checkpoints are discovered.

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[Copilot]

Pull request overview

Fixes a regression where entire explain on the default branch missed checkpoints introduced via merged feature branches by switching the default-branch scan to a full DAG walk.

Changes:

• Use repo.Log() (full DAG walk) on the default branch while keeping first-parent traversal + main filtering on feature branches.
• Extract a collectCheckpoint helper to share checkpoint→RewindPoint construction.
• Add a regression test covering merged (second-parent) checkpoint discovery on the default branch.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cmd/entire/cli/explain.go	Restores full DAG traversal on default branch and factors out checkpoint collection logic.
cmd/entire/cli/explain_test.go	Adds regression test ensuring merged-branch checkpoints are found on default branch.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.