Bind the Session's Environment on the agent host

[SaladDay]

Core binds an agent host with the Session's Environment, so production binds carry the Link resource and attach grant. Placement still never chooses the agent host; PR5c-2 does.

• Binding reads. GetSessionDevice and GetSessionExecutionBinding accept the deployment's agent host (no tenant) as the bound Runtime and return the Session's Environment (session_environment_id) beside the device's own environment_id. sessions.ExecutionDevice gains SessionEnvironmentID; environmentDeviceMatches and none placement keep reading the device's until PR5c-2.
• Bind callers. Dispatch, prepared dispatch, assignedRuntimePeer and initialization bind with the Session's Environment.
• bindLink. A bind of an agent host that holds the assignment, to an Environment without a live Link resource, fails with ErrNoLinkResource before anything is sent; callers map it to their existing unavailable outcomes. none still binds without Link fields.
• Initialization. checkInitializationDevice checks only that the listed Runtime is still the bound one; the query keeps the device-Environment rule.
• Docs. docs/runtime-protocol.md (+zh): with no live resource, Core sends the agent host no bind and the operation fails.

Tests

• TestInitializationBindsAgentHost: the agent host registered as PR5a does it and connected, with an assignment row inserted by SQL; initialization sends assignment_bind with the exact resource and an attach_grant, and without a live resource it fails before any bind.
• link_authority_test.go binds through the Worker's initialization path instead of calling peer.Bind directly.
• Focused execution, runtimegateway, sessions, sessionpg; 82 integration tests across dispatch, initialization, Environment files and Link authority; -race -count=50 on the new and Link tests; make check-names check-docs check-ci.

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