Refactor session payload drilldown

[danielftapiar]

Why

The session payload drilldown still focused on token-count interactions, but the useful next layer is session-level response analysis: commands run, command timing, and response output size by response item shape. This change stores response output and argument metrics during indexing so the UI can query aggregates without reparsing raw JSONL payloads.

Alternatives considered

• Keep the interaction list and add another nested drilldown - rejected because the current request is to remove the interaction list for now and make the session drilldown more useful directly.
• Calculate output lengths at render time from payload JSON - rejected because it would move JSON parsing back into view rendering and make startup indexing less valuable.

Impact assessment

This adds payload_events columns and bumps the payload metrics version, so existing cached payload data will be re-indexed once. The blast radius is limited to local cache schema/indexing and the payload session drilldown UI. Token summary/session/command views keep their existing data model.

Validation

• rtk go test ./...