checkpoint: dedup session-metadata mutation and test slice assertion

Extract updateSessionMetadata, the read-modify-write-blob pattern shared by replaceSkillEvents and setCompactTranscriptStart, so both express only their field mutation. Reading from the entries blob (not an in-memory copy) is kept, so sequential finalize-path mutations still see each other's changes.

Generalize the test helper assertCodexSliceIsGamma into assertCompactSliceScoped (parameterized absent/present strings), used by both the Claude and Codex slice-scoping assertions.