refactor(shell): split output buffer helpers

[cyq1017]

Refs #3958

Summary

• move shell output delta and tail buffer helpers into tools/shell/output.rs
• keep exec/wait/interact tool schemas and shell dispatch behavior unchanged
• keep approval, sandbox, timeout, and background-job behavior unchanged

Verification

• cargo fmt --check
• cargo test -p codewhale-tui --bin codewhale-tui --locked shell

[Hmbown]

Thanks @cyq1017. This looks like a clean behavior-preserving shell refactor, but it is still draft and its tracking issue is already routed to v0.8.68. I am going to leave it out of the 0.8.67 cut and pick it back up after release once it is ready for review. No harvest needed unless the branch gets stale.