[bug]: Craft cumulative growth can fall back to stale createdAt

[pitimon]

Conditional bug

Craft sessions use cumulative header totals. Timestamp selection is lastMessageAt -> lastUsedAt -> createdAt -> file mtime.

When both activity timestamps are absent or stale but createdAt exists, later cumulative growth remains in the original creation bucket even though the session file was rewritten later.

Expected policy

• First observation: activity timestamp, then createdAt, then file mtime.
• Later growth: changed trustworthy activity timestamp; otherwise current file mtime/observation time.
• Preserve per-session conversation counting, cache-token columns, eviction state, and token conservation.
• Add deterministic cross-midnight coverage for missing/stale activity timestamps.

Confirmed by provider-wide audit after Hermes #154 and Goose #158. Conditionally affected through v0.39.47.