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

No behavior change — readability, consistency, and documentation only.

• rename meta/metaErr -> summary/summaryErr in the --generate fast-fail; checkpoint.ReadCheckpoint returns a *CheckpointSummary, not *Metadata, so summary reads truer (and matches the post-load check it replaced). Also restore the spinner doc-comment's adjacency to its startSpinner call.
• document the cheap redundant metadata-only read on the non-imported --generate path; it's an acceptable tradeoff against the larger imported-path win (skipped blob prefetch + transcript load).
• comment the intentional prose-vs-JSON truncation asymmetry at the prose call site (grouped view renders up to 2*limit and notes only on a real budget hit; JSON hard-caps the flat array at limit).
• TestGetBranchCheckpoints_TruncationSignal now builds commits via the testutil git helpers (which set user identity + disable GPG signing) instead of raw go-git worktree commits, matching repo convention.