feat(core): add persistent terminal groups

[jlongster]

Summary

• add ordered session/terminal group schemas and ephemeral membership events
• persist each group independently at group:v1:<group-id>
• let callers supply group IDs and make creation idempotent
• expose atomic addItem / removeItem mutations with per-group keyed locking
• deduplicate membership while preserving item order

Verification

• bun test test/group.test.ts test/event-manifest.test.ts in packages/schema
• bun test test/group.test.ts in packages/core
• bun run typecheck in packages/schema and packages/core

Stack: 1/5; next PR adds the persistent PTY daemon and server API.

[jlongster]

Superseded by the simpler session-scoped design in #44832. The generic group service, schema, events, persistence, and HTTP API have been removed from the active stack.