[SaladDay]

Summary

The Environment now owns its lifecycle. This prepares standalone, reusable Environments; the public API follows in a later PR.

• Storage (migration 000014_first_class_environments):
  • environments gains tenant_id, its frozen configuration and session_owned, plus the creation-identity columns and constraints that standalone Environments will use.
  • sessions.environment_id names the Environment. The unique index sessions_attached_environment allows at most one live Session per Environment.
  • Preparation input (environment_setups, initial_environment_files) is keyed by environment_id.
  • Existing data is carried forward. A migration test seeds a version-13 database and checks every read after the upgrade.
• Queries: every query that joined environments to sessions now reads the Environment's own tenant, configuration and attachment. The two views are recreated.
• Lock: sessionpg.WithEnvironment locks the attached Session, if any, then the Environment, and re-checks the attachment. Workspace work uses the same lock.
• Domain types:
  • sessions.Environment gains TenantID, SessionOwned and AttachedSessionID.
  • deployment.Allocation uses AttachedSessionID and Reclaimed.
  • EnvironmentInitialization no longer carries a Session or Harness.
• Session reads: a Session's configuration.environment is composed from its Environment's row.
• Preparation without a Harness check: initialization no longer reads the Session's Harness. If the Runtime lacks the Session's Harness, the Session's input fails with runtime_preparation_failed, and the Environment is unaffected. This is the only behavior change. sessions.ProvisioningHarness is deleted.
• Egress by ownership: Session-owned Environments keep exactly today's Egress.

Docs updated in EN and zh: services/core/IMPLEMENTATION.md, contracts/agents-api/environments.md, docs/sandbox-provider.md and the diagnostics text.

Checks

• go vet ./services/core/... and scripts/check-sqlc.py.
• Unit tests for sessions, deployment, execution, api, runtimehistory and runtimeenrollment.
• Every Postgres store package.
• The integration suite, plus the new migration and Harness-availability tests.
• The official-client suite on a fresh database.
• The migration in a rolled-back transaction on a copy of a populated test database: 533 Environments, setups and files carried forward.
• pnpm --dir website check-translations.

This PR and the standalone Environments API PR get one combined blind review.

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