Add `agent_worktree_directory` to `GitSettings` for configuring where
agent worktrees are stored (default: Zed data dir). Remove `Copy` derive
from `GitSettings`/`GitContentSettings` (incompatible with String field)
and fix downstream `.as_ref().unwrap()` call sites.
Define `AgentGitWorktreeInfo` (branch, worktree_path, base_ref) and add
it to `DbThread` + `DbThreadMetadata` for persistence and session list
display.
Closes AI-33
Release Notes:
- N/A