Remove legacy group IPC after project migration

[edwin-zvs]

Goal

After project-named IPC and clients have shipped, remove the old group.* IPC method and notification surface.

Scope

• Remove legacy IPC methods:
  • group.list
  • group.create
  • group.rename
  • group.delete
  • group.set_collapsed
  • group.move
• Remove legacy notifications:
  • group/state
  • group/deleted
• Remove client helpers that only call the legacy group methods.
• Rename or remove legacy protocol payload aliases where no longer needed.
• Update tests, MCP/tool descriptions, and docs to use project terminology only.
• Consider storage/model field cleanup separately if persisted group_id compatibility still needs a staged migration.

Prerequisites

• Project IPC compatibility PR has merged.
• TUI/web/MCP/adapter callers use project.* methods and project/* notifications.
• Any released clients that still require group.* support are either updated or explicitly no longer supported.

Notes

This is intentionally a cleanup follow-up, not part of the compatibility PR. The first migration should keep group.* aliases so existing clients do not break.