feat(mcp): tools to create session groups and file sessions under them

[blpsoares]

Summary

• MCP tools so an assistant can organise the sessions it starts: agentistics_session_groups (list), agentistics_session_group_create (optionally files sessions in the same call) and agentistics_session_group_edit (add / remove / rename / delete), over new /api/session-groups routes.
• Groups are addressed by id or name, sessions by managed id, conversation id, exact title or unique id prefix. A reference that matches nothing (404) or two things (409) is refused, and a batch with a bad session changes nothing.
• Filing follows the sidebar's rules: a session belongs to at most one group (adding moves it) and is unpinned.

Motivation

Requested: let LLMs create the groups and put the sessions they start inside, to keep the fleet organised.

Changes

• @agentistics/core sessionGroups.ts: the pure planners move here (planGroupOp, resolveGroupRef, resolveSessionForGroup, sessionIdentityKey); the web module re-exports them, so both surfaces share one rule.
• server/sessions/session-groups-web.ts + routes in index.ts + capability-guard.ts (localShell).
• preferences.ts: updatePreferences, an atomic read-modify-write (the browser writes the same key).
• packages/mcp/agentistics-mcp.ts, docs/mcp.md, CLAUDE.md.

Test plan

• tsc + tests (pre-commit): core planners, server module with an in-memory store, and concurrent updatePreferencesAt (20 writers, none lost)
• Real server on an isolated data dir: create / refusals (404, 400) / rename by name / delete by name; a seeded registry filed by title and by id prefix, stored with the right identity keys
• The MCP server over stdio: 3 tools listed, create + add by group name and session title moves a session between groups, a bad group returns the refusal message

Notes

The browser adopts a change made through the MCP when its tab regains focus (the same refresh every shared preference uses).

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