Fix lint findings

golangci-lint v2.11.4 was unhappy across errcheck, exhaustive, gocritic, inamedparam, ireturn, maintidx, noctx, perfsprint, revive, and wrapcheck. Mostly mechanical:

• Test helpers (initSHA1, initSHA256, mustTranslator) collapse the '_ :=' patterns into t.Fatalf-on-error wrappers.
• exec.Command -> exec.CommandContext(ctx, ...); ctx threaded into runChecks and signBranchTips.
• refs.ForEach return value checked instead of discarded.
• if/else chain in runChecks rewritten as switch.
• ireturn for openSource/normalizeAuth annotated, since both return shared transport interfaces by design.
• maintidx on Run annotated; the function is a phase orchestrator, splitting it would obscure the pipeline.
• exhaustive switches on plumbing.ObjectType annotated; the unhandled cases (OFSDelta/REFDelta/AnyObject/InvalidObject) can't reach a resolved storer.
• Errors from io.ReadAll/MemoryObject.Reader/bufio.Flush/fmt.Fprintln/ auth.Method.Authorizer wrapped with fmt.Errorf.
• Constant 'max' renamed to package-level 'previewMax' to stop shadowing the builtin in two places.
• Named parameter added to interface methods that lint flagged.
• Two fmt.Sprintf calls replaced with string concatenation.