feat(subagent): worktree.bgIsolation: none — allow background sessions to edit working copy directly

[bug-ops]

Context

Deferred from spec specs/parity-claude-code-3918/ (PR #4652, issue #3918 task T3.2).

Claude Code v2.1.143 added worktree.bgIsolation: none to allow background sessions to edit the working copy directly without entering a worktree (for repos where worktrees are impractical).

Scope

Add worktree.bg_isolation: none | worktree config option. Blocked by #4655 (worktree subsystem).

Blocked by

#4655 (worktree.baseRef / worktree subsystem)