Remove dead user management code from SDK

[krisrowe]

Problem

gapp/admin/sdk/users.py and parts of gapp/admin/sdk/tokens.py implement user management by directly reading and writing GCS bucket files under an auth/ prefix. This was the original pattern when gapp injected an auth wrapper (gapp_run) into deployed containers.

Solutions now use mcp-app's user-identity middleware for auth, which:

• Validates JWTs directly in the app
• Stores user records via its own UserAuthStore (filesystem-based under data/users/)
• Serves a REST admin API at /admin/users and /admin/tokens

The gapp user management code doesn't interact with any of this. gapp users list returns 0 users for a deployed solution that has active users, because it's looking at auth/ bucket files that don't exist in the new pattern.

User management tooling is being built in mcp-app instead (see echomodel/mcp-app#5).

What to Remove

• gapp/admin/sdk/users.py — all functions: register_user, list_users, get_user, update_user, revoke_user, and GCS helpers (_gcs_path, _object_exists, _write_credential, _read_credential_full, _email_hash)
• gapp/admin/sdk/tokens.py — references to users.py helpers (_email_hash, _gcs_path, _object_exists, _read_credential_full, _write_credential). create_token and revoke_tokens check user existence via GCS and need to be removed or reworked
• gapp/admin/sdk/tokens.py — create_status_token and _get_signing_key may still be needed by mcp_status.py for health checks. Audit before removing.
• CLI commands that wrap these SDK functions (e.g., gapp users list, gapp users register, etc.)
• MCP tool handlers that wrap these SDK functions (gapp_users_list, gapp_users_register, gapp_users_update, gapp_users_revoke, gapp_tokens_create, gapp_tokens_revoke)
• build/lib/gapp/admin/sdk/tokens.py — stale build artifact with same dead code
• Related tests if any exist

What to Keep (audit first)

• create_status_token — used by mcp_status.py for health-check probing. This mints a short-lived JWT with scope: "status" to bypass auth on health checks. Likely still needed.
• _get_signing_key — reads signing key from Secret Manager. Still needed by create_status_token and potentially by future admin client code.

Context

• User management now belongs in mcp-app: Add CLI and MCP tools for user admin mcp-app#5
• The gapp_run auth wrapper (run/gapp_run/) is also legacy but is a separate cleanup item