The CLI runtime emits `github.copilot.time_to_first_chunk` (seconds) on
chat spans, while the foreground extension uses
`copilot_chat.time_to_first_token` (ms). The SQLite store only checked
the foreground attribute, leaving ttft_ms NULL for CLI background agent
spans.
Add a `_ttftMs()` helper that coalesces both attributes with unit
conversion (seconds → ms) so the denormalized column is always populated.
8d134899f3