Scope background activity to the owning session

[cbusillo]

Summary

• scope model-facing agent get/list/cancel/wait paths to the owning session id
• remove the process-global background-shell completion submission sender and enqueue completion wakeups on the originating session
• add a manager regression covering cross-session agent list/get/cancel behavior, including archived agents

Validation

• cargo test -p code-core model_facing_agent_queries_are_session_scoped
• cargo test -p code-core agent_status_updates_include_only_owned_agents
• cargo test -p code-core --test agent_completion_wake concurrent_same_repo_sessions_only_receive_owned_agent_status
• cargo test -p code-core --no-run
• ./build-fast.sh

Fixes part of #175.