checkpoint: name Write requests by scope (Session / Checkpoint*)

Rename the Write request union to noun-based, scope-explicit names — the verb is already Write, and the tier (session vs checkpoint) should be visible in the type: WriteSession -> Session (session-level: write/replace a session) BackfillTranscript -> SessionTranscript (session-level) BackfillSummary -> SessionSummary (session-level) BackfillAttribution -> CheckpointAttribution (checkpoint-level: root combined attribution)

The old BackfillAttribution name hid that it targets the checkpoint root's combined attribution across all sessions, not a single session — CheckpointAttribution makes the one checkpoint-level write explicit alongside the three session-level ones.