Tasks: Newline-safe worktree registration

• T001 [FR-001] Reproduce a registered newline path as unregistered in crates/signaltty-server/src/git.rs.
• T002 [FR-001] [FR-003] Parse NUL-delimited worktree porcelain in is_worktree_registered, comparing non-UTF-8 fields lossily.
• T003 [FR-002] Keep the existing add/remove registration guard and fail-closed Git errors.
• T004 [FR-001] [FR-002] [FR-003] Complete review and full verification before publishing.