feat(attach): number sessions in list, fix a reattach control-char leak

[jayhesselberth]

Summary

• `list`'s human table gets a leading `#` column (1-based position); `attach` and `cancel` now accept that position in place of a JOBID or NAME (`sinteractive attach 1`), falling back to it only when no running session's job id actually matches the number.
• Fixes `attach`/launch-and-attach/`__attach` querying the terminal's background colour live right before exec'ing into the zellij client that takes over the same terminal — on a laggy or just-reconnected ssh link, a late OSC 11 reply was landing as literal control characters in the freshly attached pane instead of at a shell prompt.

Test plan

• `cargo fmt --all --check`
• `cargo clippy -p sint-core -p sint --all-targets -- -D warnings`
• `cargo test -p sint-core -p sint` (231 tests)