sessions: refine project grouping in agent sessions view

[osortega]

Summary

This updates the dedicated agent sessions window behavior when sessions are grouped by project.

• sort project groups alphabetically instead of letting the created/updated sort mode drive group order
• keep pinned sessions at the top without rendering a separate Pinned section header
• include the repository name inline for pinned sessions so they still show project context before the timestamp
• keep the created/updated sort mode scoped to ordering sessions within each project group
• continue to keep Other above Archived, with archived sessions always last

Why

When grouping by project, the created/updated setting should only affect session ordering inside each group. Project group names should stay stable and alphabetical, and pinned sessions should stay prominent without losing project context.

Validation

• npm run compile-check-ts-native
• node --experimental-strip-types build/hygiene.ts src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsViewer.ts

[Copilot]

Pull request overview

Refines how the Agent Sessions standalone window groups sessions by project/repository, aiming for stable alphabetical group ordering while keeping pinned sessions prominent without a separate “Pinned” header.

Changes:

• Render pinned sessions as top-level items (no “Pinned” section header) in repository grouping.
• Sort repository/project groups alphabetically (instead of inheriting global created/updated sort order).
• Add inline repository context to pinned sessions’ status/time label when grouped by repository.