[FEATURE] MCP tools to manage custom session groups in CCD sidebar

Problem

The CCD session-management MCP server exposes list_sessions, search_session_transcripts, and archive_session. It does not expose any tools for reading or writing the custom user groups that the sidebar supports — the groups a user creates manually for organizing sessions, distinct from the auto-by-project grouping discussed in #59982.

Use case

A single project (cwd) that accumulates many sessions across distinct domains. I currently have one cwd with ~50 sessions spanning legal/compliance, product/API, business development, infrastructure/meta, and knowledge-management work. Auto-by-project grouping puts them all in one bucket. Custom groups in the sidebar would solve this, but only if there's a way to create and assign them programmatically — manually dragging 50 sessions one-by-one is the bottleneck, and the categorization itself is exactly the kind of work an agent should do.

I asked an agent to help me organize. It was able to read all 50 session titles via list_sessions, propose 6 domain groups with per-session assignments, and produce a clean per-group plan — but had to hand the plan back to me as a manual click checklist because there's no MCP tool to actually do the assignment.

Asks

1. Add a group field (or equivalent) to the list_sessions return shape so agents can see existing assignments.
2. Add MCP tools to manage custom groups, e.g.:

• list_session_groups
• create_session_group(name)
• assign_session_to_group(session_id, group_name)
• remove_session_from_group(session_id, group_name)

Why MCP and not just settings.json

#59982 (closed) covered cwd → group static mapping. That works when sessions live in different cwds. It doesn't help when many sessions live in the same cwd and need to be subdivided by topic. Programmatic MCP access lets an agent batch-categorize from session titles or transcript content, which is hard to do via static config.

Environment

Claude Code Desktop on macOS (Darwin 25.5.0), Opus 4.7.