[P1] Define a consistent activity and run history model

[Gupta91]

Problem

Agent runs, browser sessions, review sessions, and timeline entries overlap conceptually but are modeled separately.

Expected Behavior

Define one user-facing activity/run model for history, status, navigation, and details.

Acceptance Criteria

• Document the relationship between agent runs, browser sessions, review sessions, and timeline entries.
• Identify which UI surfaces should show activity history.
• Propose migration steps that preserve existing data.