Adds `manager.flags.{discuss,plan,execute}` config keys so users can
configure flags that /gsd:manager passes to each step when dispatching.
For example, `manager.flags.discuss: "--auto --analyze"` makes every
discuss dispatched from the manager include those flags.
Closes #1400
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>