feat(client,mcp): shifts vertical

[dougborg]

Summary

Build a client.shifts helper + MCP tools covering Front's shift /
on-call schedule surface (10 endpoints).

Status

Deferred — filed as part of the #16 workspace-admin partition.
Schedule management is admin work. Pick up when a concrete agent use
case appears — likely "is teammate X on call right now?" or
"who's on call for inbox Y?" — rather than admin-side shift creation.

Scope

• Generated modules under frontapp_public_api_client/api/shifts/:
  • Reads: list_shifts / list_team_shifts / list_teammate_shifts /
    list_shifts_teammates / get_shift
  • Mutations: create_shift / create_team_shift /
    update_shift / add_teammates_to_shift /
    remove_teammates_from_shift
• New helpers/shifts.py + client.shifts lazy property.
• MCP tools — reads + mutation tools with confirm_or_preview.
  A who_is_on_call(inbox_id) tool is the concrete agent value — open
  question whether to expose the raw shift catalog or only the
  derived "who's on call right now" lookup.
• Tests + docs.

Umbrella

Part of #16 (workspace admin partition). Tier 3.