fix(core): retain interrupted shell output

[rekram1-node]

Summary

• separate stopping a shell from deleting its retained state
• retain partial output when foreground shell jobs are interrupted
• keep explicit shell removal semantics unchanged

Validation

• bun test test/tool-shell.test.ts in packages/core
• bun test test/shell.test.ts in packages/core
• bun typecheck in packages/core
• git diff --check

[rekram1-node]

Closing in favor of tracking the broader interrupted-tool result design in #39565.