Forker launcher uses fragile substring matching for agent detection

## Description
`launcher.go` uses `strings.Contains(agent, \"o4\")` which matches unintended names like \"proto4\". The default fallback to Claude for unknown agents is silent.

## Fix
Use prefix matching or an explicit map and return an error for unrecognized agents.

## Location
`internal/forker/launcher.go:22-27`