Managers can't choose the model when creating a team or starting members #1839

Summary

Task delegation already lets the caller pick the model (propose_task's model param; delegate_door.go speaks of "the models the person asked it to work with"). Team creation has no equivalent: neither the person making a team from the wall nor the manager starting members can say which model anyone runs on. Members inherit whatever the profile gives them.

Current behavior

• Person-side: the wall's new-team card (internal/tui3/wall.go wallNameCard, opened by teamMenuNewTeamIn in internal/tui3/teammenu.go) names the team and marks member tiles. No model step.
• Manager-side: team_start (internal/session/team.go:669, prompt guidance at :1361 — "Hand real work to members with team_send, or team_start for a new member") takes a brief but no model.
• Storage: teams.Member (internal/teams/teams.go:20) carries Key/File/Where/Word/Handle/Home/Started/HandleBy — no model field. Team.Settings (teamsettings.go; knownFields: questions_up, cap_usd_day, depth_limit, sub_share, wake) are delegation overrides only — no model there either.

Why it matters

A manager fanning out a team cannot staff it by model: run a cheap model for scouts/readers and a stronger one for the implementers, or pick the model the manager itself runs on. Tasks can do this per-proposal; teams, the coarser and longer-lived unit, cannot.

Ask

Mirror propose_task's model param for teams:

1. team_start accepts a model for the member it opens.
2. A team-level default model in Team.Settings that members inherit down the parent chain, the same way cap_usd_day/depth_limit/sub_share inherit — unset fields fall back to the profile default.
3. Optionally, the wall's new-team card exposes the default when the person creates a team.

Model names should resolve the way tasks resolve theirs today.