Hoist the existing stderrTail ring-buffer (previously codex-only) into
a shared pkg/agent helper so every Backend that supervises a child CLI
can include the last ~2 KB of that CLI's stderr in Result.Error. Wire
the claude backend through the same path.
Motivation: claude on Windows occasionally exits with a non-zero status
after ~5–8 minutes of a single long-running tool_use, and right now the
daemon only reports "claude exited with error: exit status 3" /
"exit status 0x80000003" — useless for root-causing V8 aborts, Bun
panics, native-module OOMs, or any other CLI-side crash. With the tail
attached, the failure message carries the real signal (panic line, V8
assertion, stderr-printed HTTP error) all the way into the task row's
error field that users see in the API.
Renames withCodexStderr to withAgentStderr(msg, label, tail) so the
helper is self-documenting across providers.
