Remove the separate `AgentGitWorktreesFeatureFlag` and replace all its
checks with the existing `AgentV2FeatureFlag`. The git worktree
functionality is now gated behind the same flag as agent v2.
Changes:
- Removed `AgentGitWorktreesFeatureFlag` from `feature_flags`
- Replaced all `has_flag::<AgentGitWorktreesFeatureFlag>()` checks with
`has_flag::<AgentV2FeatureFlag>()`
- Simplified toolbar condition that was redundantly checking both flags
- Updated tests accordingly
Closes AI-73
Release Notes:
- N/A