feat(core): add entity module stubs, migration runner, and Storage wrapper

[geoffjay]

Sets up the entity/migration/storage scaffold in crates/core that all downstream entity issues (#237, #238, #239) depend on.

• entity/mod.rs — declares four sub-modules: user, organization, membership, session
• entity/{user,organization,membership,session}.rs — minimal DeriveEntityModel stubs (id-only, correct table names) that compile and are ready to be filled in
• migration/mod.rs — empty Migrator implementing MigratorTrait, ready for future migration files
• storage.rs — Storage struct wrapping DatabaseConnection; runs Migrator::up on construction; Clone + Send + Sync
• Cargo.toml — adds sea-orm, sea-orm-migration, async-trait workspace/direct deps
• lib.rs — declares entity, migration, storage modules

Closes #241

[geoffjay]

This change is part of the following stack:

• feat(core): scaffold agentd-core service #1058
  • feat(core): add entity module stubs, migration runner, and Storage wrapper #1059 ◀
    • feat(core): implement initial sea-orm migration for core service schema #1060
      • feat(core): define SeaORM User entity with has_many relations #1061
        • feat(core): define SeaORM Session entity with belongs_to User #1062
          • feat(core): define Organization and Membership entities with many-to-many relations #1063
            • feat(core): add SeaORM integration tests for entity relations #1064
              • feat(core): add OrganizationStorage and MembershipStorage #1066
                • feat(core): add UserStorage with argon2 password hashing and username migration #1067
                  • feat(core): add authentication endpoints with SQLite-backed sessions #1068
                  • feat(core): add TenantContext middleware and active organization endpoint #1069
                  • feat(core): add user and organization management API endpoints #1070
                  • feat(cli): add auth and org commands for core service #1071
                  • feat(core): add API gateway proxy for downstream service routing #1072

_{Change managed by git-spice.}

[geoffjay]

Review: feat(core): add entity module stubs, migration runner, and Storage wrapper

Clean scaffold. All four entity stubs, the migration runner, and the Storage wrapper follow the orchestrator's established patterns exactly. No correctness issues.

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Stack context

issue-241 (#1059) is stacked on issue-212 (#1058), which was just approved and is in merge-queue. This branch cannot merge until #1058 lands. Once #1058 merges to main, this branch will need a restack to update its base — adding needs-restack accordingly.

Ordering note for the #240 worker: The triage plan defined the chain as #212 → #240 → #241, but this PR implements #241 directly on top of #212, ahead of #240. That's fine as a scaffold-first approach, but it changes what #240 needs to do: instead of creating migration/mod.rs from scratch, the #240 worker must stack on top of this branch and update the existing migration/mod.rs to register the actual migration file(s). Creating a fresh migration/mod.rs in a branch off main would conflict with this PR.

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Entity stubs — correct

All four stubs follow the same minimal pattern:

• #[sea_orm(table_name = "...")] uses the correct plural snake_case table names: users, organizations, memberships, sessions ✅
• auto_increment = false on the UUID id PK ✅
• Empty Relation enum and impl ActiveModelBehavior for ActiveModel {} ready for #237/#238/#239 to fill in ✅
• Doc comments reference the downstream issues that will complete each entity ✅

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migration/mod.rs — correct scaffold

pub use sea_orm_migration::prelude::* + empty vec![] migration list matches the orchestrator's migration/mod.rs pattern exactly. The doc example showing Migrator::up(&db, None).await? is accurate. ✅

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storage.rs — correct

Matches AgentStorage pattern:

• Clone via DatabaseConnection (pool handle, cheap clone) ✅
• Runs Migrator::up on construction before returning ✅
• db() accessor for entity-specific operations ✅
• anyhow::Result for error propagation ✅

The doc example references agentd_common::storage::{get_db_path, create_connection} — consistent with how AgentStorage is initialized. ✅

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Dependencies — correct

• sea-orm = { workspace = true } and sea-orm-migration = { workspace = true } ✅
• async-trait = "0.1" matches orchestrator's direct pinned dep (it's not in the workspace table) ✅

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No tests

Acceptable for this scaffold — Storage::new has no logic beyond Migrator::up (which returns immediately with an empty migration list), and the entity stubs have no fields to exercise. Tests belong in the downstream issues (#237, #238, #239, #242).

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Summary

Correct scaffold, consistent with existing patterns. Approving. Parent PR #1058 must merge first; #240 worker should stack on top of this branch.