Finish the crew settings review: file holds, team dials and subscriptions each kept, folded or deleted #8732

Left open from #7668 (pull #8690, 006777db). The pull reviewed mode, direction, ceiling, decisions, plan nods and outsiders, but not file holds, team dials or subscriptions. The owner asked for fewer settings, and for none that hold work silently (#7636).

Done looks like:

• [x] Each of the three has a decision (keep, fold into a seat's number, or delete), with one line of why, in the table in docs/features/seat-slots.md.
• [x] What is folded or deleted is gone from Settings › Team, the MCP crew tool and the API, and its stored value is migrated or ignored.
• [x] A kept setting that holds work says so on the line where the work waits, not in a banner.
• [x] Tests for each removal or fold.

Files: src/web/studio/crew.ts, src/http/mcp/tools/crew.ts, docs/features/seat-slots.md, tests beside them Risks: if a deleted dial is still read by the dispatcher, work can start that was meant to wait. Grep every reader first.

Reviewed pull #8772 at its head: ticked Each of the three has a decision (keep, fold into a seat's number, or delete), with one line of why, in the table in docs/features/seat-slots.md.; What is folded or deleted is gone from Settings › Team, the MCP crew tool and the API, and its stored value is migrated or ignored.; A kept setting that holds work says so on the line where the work waits, not in a banner.; Tests for each removal or fold..