Tasks: CLI pane identity and working directories

• T001 Reproduce named attach remaining open after close and exit.
• T002 Reproduce input redirection after renaming and reusing a pane name.
• T003 Reproduce relative cwd launching pwd in the server directory.
• T004 Retain the resolved pane ID in crates/signaltty-cli/src/attach.rs.
• T005 Resolve explicit cwd in crates/signaltty-cli/src/main.rs.
• T006 Verify real CLI regressions in crates/signaltty-cli/tests/cli.rs.
• T007 Document cwd semantics in docs/08-ipc.md.
• T008 Run scripts/verify.sh full and review the diff before publishing.