feat(filter): expose branch filters in clients

[prateek]

Exposes branch filtering in the user-facing surfaces.

The CLI and MCP tools accept project-scoped branch filters. The web app can load branch metadata, keep branch filters in the URL, show branch controls in the session sidebar, and pass the selected branch through to usage requests.

Stack context:

• Tracking issue: Add git branch as an analysis dimension kenn-io/agentsview#928
• Base PR, foundation: feat(filter): add branch filter foundation kenn-io/agentsview#927
• This fork-internal PR: PR 2, filter surfaces
• Siblings: feat(activity): add branch rollups #2 and feat(usage): group usage by branch #3

This PR targets PR 1's branch. Once PR 1 lands upstream, this branch can be restacked onto main and opened against kenn-io/agentsview without waiting for the activity or usage follow-ups.