Remove legacy groups/<project-id>.json project metadata migration

[edwin-zvs]

Goal

After the project metadata layout has shipped and users have had time to migrate, remove the legacy read/migration path for <AGENTD_DATA_DIR>/groups/<project-id>.json.

Context

Issue #228 / PR #234 moves project metadata to:

<AGENTD_DATA_DIR>/projects/<project-id>/meta.json

and keeps project memory beside it:

<AGENTD_DATA_DIR>/projects/<project-id>/memory.md

For compatibility, PR #234 still reads legacy groups/*.json, writes the equivalent projects/<id>/meta.json, and removes the legacy file best-effort.

Scope

• Remove legacy groups_root / group_path usage once no longer needed.
• Remove fallback scanning of <AGENTD_DATA_DIR>/groups/*.json from project loading.
• Remove tests that assert legacy group metadata migration.
• Update docs/comments to describe only projects/<id>/meta.json.
• Confirm project deletion still removes only meta.json and preserves memory.md.

Prerequisites

• PR feat(memory): load markdown global and project memory #234 has shipped in at least one release/build used by existing users.
• Existing deployments have had an opportunity to start once and migrate legacy group metadata.