fix: rita returns "error: -13" when the next program in a pipeline closes and...

[Roshan931]

Fixes #803

Root cause

Unhandled SIGPIPE kills rita when its stdout pipe closes

Changes

• Install a SIGPIPE notification handler in main so writes to a closed stdout/stderr return EPIPE instead of terminating rita with signal 13, and add a regression test that runs a child process whose stdout pipe is closed mid-write and asserts it still exits cleanly.