Place Sessions on the agent host

[SaladDay]

Every Session now runs on an agent host. The sandbox serves its Link resource through oac-sandbox-io, and no Runtime runs inside a sandbox or on a tenant device.

• Placement: one path. A Session is bound to the first ready agent host, and a Session with an Environment only while its Link resource is Serving. The initialization scanner places an unassigned Session. ListAgentHosts replaces ListExecutionDevices, and BindSessionDevice requires an agent host and, with an Environment, a live resource.
• self_hosted: EnrollRuntime records the enrollment (first key wins, 409 for another key) and returns {link_url, resource}. The Linux launcher supervises oac-sandbox-io --bootstrap-file, which ships as a second binary in the installer bundle. darwin and windows return a typed unsupported error before enrolling.
• Connection status: follows the Serving rule (RuntimeConnected, enrolled_at, bound_key_id); last_seen_at is gone from the route, OpenAPI and Web.
• Revocation and rotation: one revocation pass over observed transitions. Rotation advances the resource generation and the assignment epoch in one statement.
• Compute: quiesce and resume target the assigned agent host and wait, bounded, for Serving. The in-sandbox resume fallback is deleted.
• Migration: the upgrade refuses while a live Session is bound to a Runtime that is not an agent host.

The guest connect --remote/--environment-id/--credential-file flow and the application-managed E2B example that uses it stop working here; PR6 deletes them with the rest of the guest path.

Checks: go vet ./...; unit tests of the changed packages; the changed sessionpg and integration tests (-run); -race -count=50 on the gateway, enrollment, launcher and Link-authority tests; darwin and windows builds of the daemon; make openapi (no diff); make check-names check-docs check-ci; Web vitest and tsc; one end-to-end native-onboarding-smoke.mjs run.

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