Bug: --matrix prompt overridden by required -p flag

[c-h-]

What happened

agentctl launch --matrix matrix.yaml fails with required option '-p, --prompt <text>' not specified. Passing -p placeholder satisfies the CLI but the placeholder overrides the prompt defined in the matrix YAML file.

Expected

--matrix should make -p optional, using the prompt from the YAML file.

Workaround

Launch lanes individually with the correct prompt instead of using --matrix.

Impact

Matrix sweep feature can't be used with per-matrix prompts — defeats the purpose of a single CLI expression for sweep launches.