feat(tui): show live background shell output

[Hmbown]

Summary

• Harvests the live shell-output intent from feat(tui): live shell output during execution - show real-time progress #2048 as a narrow maintainer patch.
• Shows bounded stdout/stderr tails inline on background shell Exec cards keyed by the exact shell task_id.
• Uses existing ShellManager::list_jobs() snapshots, so the UI does not consume output deltas and does not match by command text.
• Keeps final tool output authoritative: once an Exec cell has final output, stale live tails are ignored.

Credit

Thanks @donglovejava / dongchao for #2048 and the original live shell-output direction. This PR intentionally reimplements the safe slice on the current v0.9 shell/job primitives rather than cherry-picking the older PR.

Verification

• cargo test -p codewhale-tui shell_live_output --locked
• cargo test -p codewhale-tui exec_cell_ --locked
• cargo clippy -p codewhale-tui --bin codewhale-tui --locked -- -D warnings
• cargo fmt --all -- --check
• ./scripts/release/check-versions.sh
• git diff --check

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