fix(core): show stdout tail in exec() failure message when stderr is empty

[Larslllllll]

Problem

exec()'s non-zero-exit error message picks its tail with:

const tail = stderr.trim().split('\n').pop() ?? stdout.trim().split('\n').pop() ?? '';

But ''.trim().split('\n') is [''], so .pop() returns '' — not undefined. The ?? fallback to stdout never fires.

Many CLIs write their diagnostics to stdout (and nothing to stderr), so failures looked like:

sh -c echo useful-diagnostic-on-stdout; exit 3 failed (exit 3): 

— the actual diagnostic was dropped from the error the user sees.

Verified before this patch

> exec('sh', ['-c', 'echo useful-diagnostic-on-stdout; exit 3'])
Error: sh -c ... failed (exit 3):         // stdout content missing

Fix

Pick the last non-empty stderr line; fall back to the last non-empty stdout line.

Error: sh -c echo useful-diagnostic-on-stdout; exit 3 failed (exit 3): useful-diagnostic-on-stdout

Tests

• empty stderr → stdout tail appears in the message
• multi-line stderr → last non-empty stderr line wins