fix(llm): surface claude-cli errors reported on stdout

[davetothek]

The claude CLI reports usage-limit and auth errors on stdout with an
empty stderr while exiting non-zero. _call_claude_cli / _call_llm only read
proc.stderr, so these failures surfaced as a bare claude -p exited 1: with
no indication of the actual cause — an unrecoverable usage limit looked
identical to any other non-zero exit.

Fall back to the stdout tail when stderr is empty, so the surfaced error carries
the real message. Adds tests/test_llm_errors.py covering both paths (prefer
stderr when present; fall back to stdout otherwise).