[Suggestion] Add project and agent analytics #37

Is your feature request related to a problem? Please describe.
The usage dialog is global and does not explain which project, directory, workspace, agent, or subagent is responsible for usage.

Describe the solution you'd like
Add filters and breakdowns for project, directory, workspace, agent, provider, model, and root versus descendant sessions, including subagent cost share.

Additional context
Execution order: 5, after #36. Preserve the current global view as the default and handle session-list pagination.