[BUG] `--agent <unknown-name>` is silently ignored instead of erroring

Environment

• Claude Code: 2.1.168
• OS: macOS 15.7.7 (Intel, Darwin 24.6.0)

Steps to reproduce

claude -p "Reply with exactly OK" --model haiku --agent nonexistent-agent-xyz
echo $?

Expected

A hard error (or at minimum a warning on stderr) that the agent nonexistent-agent-xyz is not defined — matching the behavior of other flags that validate their values (e.g. --permission-mode rejects invalid choices).

Actual

Exit code 0, the prompt runs against the default session configuration, and no warning is emitted. The flag is silently ignored.

Working comparison

claude -p "Reply with exactly OK" --model haiku \
  --agents "{\"reviewer\": {\"description\": \"Reviews code\", \"prompt\": \"You are a code reviewer\"}}" \
  --agent reviewer    # works as documented

Why it matters

A typo in --agent (or a missing --agents/settings definition in CI) silently runs with the wrong system prompt and toolset instead of failing — exactly the kind of misconfiguration that should fail loudly in scripted -p usage.