Add user model and SQLite storage to core service

[geoffjay]

Context

The core service (#211) requires a user model for multi-user authentication support. This builds on the scaffolded core crate (#212).

Acceptance Criteria

• Create User model struct with fields:
  • id (UUID, primary key)
  • username (TEXT, unique, not null)
  • email (TEXT, unique, not null)
  • full_name (TEXT, not null)
  • password_hash (TEXT, not null)
  • active_organization_id (UUID, nullable, foreign key to organizations)
  • created_at (TEXT/datetime)
  • updated_at (TEXT/datetime)
• Create UserStorage with SQLite schema initialization
• Implement CRUD operations: create, get_by_id, get_by_username, get_by_email, update, delete
• Implement list_paginated using agentd_common::types::PaginatedResponse
• Add password hashing using argon2 crate (hash on create, verify on login)
• Add set_active_organization method
• Add unit tests for all storage operations
• Use agentd_common::storage::{get_db_path, create_pool} for DB setup

Relevant Files

• crates/orchestrator/src/storage.rs — reference pattern for storage implementation
• crates/common/src/storage.rs — shared storage utilities
• New: crates/core/src/models/user.rs, crates/core/src/storage/users.rs

Dependencies

• Blocked by Scaffold the core service crate #212 (scaffold core crate)