explain: address trail review comments (naming, test helpers, docs)

[computermode]

https://entire.io/gh/entireio/cli/trails/696

What

Follow-up to PR #1560: addresses the trail review comments on the explain changes. Readability, consistency, and documentation refinements — no behavior change.

How

• Naming: renamed meta/metaErr → summary/summaryErr in the --generate fast-fail. checkpoint.ReadCheckpoint returns a *CheckpointSummary, not *Metadata; .Imported exists on both so it compiled, but summary reads truer and matches the post-load check it replaced. Also restored the spinner doc-comment's adjacency to its startSpinner call.
• Tradeoff doc: documented the cheap, redundant metadata-only read on the non-imported --generate path — acceptable against the larger imported-path win (skipped blob prefetch + transcript load).
• Asymmetry doc: added a comment at the prose list call site explaining the intentional prose-vs-JSON truncation difference (grouped view renders up to 2*limit and notes only on a real budget hit; the JSON path hard-caps the flat array at limit).
• Test consistency: TestGetBranchCheckpoints_TruncationSignal now builds commits via the testutil git helpers (user identity + GPG-signing disabled) instead of raw go-git worktree commits, matching repo convention.

Verification

mise run fmt + mise run lint clean (0 issues); full cmd/entire/cli unit suite green.

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Note

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[Copilot]

Pull request overview

Refines entire explain implementation details and tests based on prior trail review feedback, focusing on naming clarity, documentation precision, and aligning tests with repository git-test conventions (intended to be no behavior change).

Changes:

• Renamed meta/metaErr to summary/summaryErr for the --generate metadata-only fast-fail path and expanded the rationale comment.
• Repositioned the spinner pipeline comment to sit adjacent to startSpinner.
• Updated TestGetBranchCheckpoints_TruncationSignal to build commits via testutil helpers and added documentation explaining the intentional truncation asymmetry between prose and JSON paths.

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	Improves naming and adds clarifying comments around imported fast-fail, spinner behavior, and truncation semantics.
cmd/entire/cli/explain_test.go	Switches test commit creation to repo-standard testutil helpers and clarifies intent in test comments.