[codex] Fix project hierarchy grouping

[jamesrochabrun]

Summary

Fixes project/session grouping so sessions always display under the root repository that owns the worktree. This keeps root projects as the section headers and places sessions from linked worktrees, including worktree subdirectories, under that root project.

Details

• Adds shared project hierarchy path resolution for root repo/worktree grouping.
• Canonicalizes added Claude and Codex repository paths so adding a linked worktree stores the owning root repo.
• Updates monitored and selected session grouping views to use the shared root-project resolver.
• Adds regression coverage for worktree paths, worktree subdirectory paths, sibling prefix boundaries, and monitor-service root canonicalization.

Validation

• xcodebuild test -project app/AgentHub.xcodeproj -scheme AgentHub -destination 'platform=macOS' -only-testing:AgentHubTests/ProjectHierarchyResolverTests -only-testing:AgentHubTests/SessionMonitorRepositoryRootTests -only-testing:AgentHubTests/CLISessionMonitorServiceHistoryTests