Add session execution binding and thin PG Phase 2 substrate #70

Summary

Separate WHERE a session runs from WHO owns it by introducing a first-class execution binding abstraction, then layer a thin Phase 2 ownership substrate (owners, leases, fencing tokens) on top of Phase 1 PostgreSQL persistence.

This work covers ADR-0014 and ADR-0013. The first executable slice is captured in the task packet below.

Why now

SessionManager currently conflates three concepts inside the Session object:

1. Durable session metadata (id, provider, model, policy, tape_id).
2. Execution/workspace binding (repo_path, tool filesystem scope, shell working directory).
3. Current runtime owner (task, approval_event, event_queues, runtime_pipeline, runtime_ctx, runtime_adapter).

These change on different timelines. Without a binding boundary, repo_path cannot represent a cloud workspace without fragile string parsing, and multi-instance HTTP deployment has no answer to stale owners continuing to drive runtime state after failover.

Scope

1. Execution binding abstraction

   • ExecutionBinding base class with LocalExecutionBinding and CloudWorkspaceBinding.
   • BindingResolver protocol and DefaultBindingResolver.
   • Integrate binding into Session metadata round-trip and runtime hydration.

2. Thin Phase 2 ownership substrate

   • PostgreSQL session_owners table with atomic acquire/renew/release.
   • App-layer SessionOwnerStore with fencing token validation.
   • Owner checks in SessionManager.run_agent, _restore_checkpoint, and close_session.

Non-goals

• Cloud workspace tool implementations (only the binding skeleton).
• In-flight turn resume after owner loss (new owners rebuild cold state only).
• Brokered event routing or sticky routing hardening (acceptable for now).
• HTTP server endpoint changes for binding storage (follow-up slice).

References

• ADR-0014: docs/adr/0014-separate-session-execution-binding-from-runtime-ownership.md
• ADR-0013: docs/adr/0013-define-phase2-multi-instance-session-ownership-for-pg-http-sessions.md
• Implementation plan: docs/superpowers/plans/2026-04-19-session-execution-binding-and-thin-phase2.md
• Task packet (first slice): .opencode/prompts/session-execution-binding-phase2-task-packet.md

Acceptance criteria

• test_local_binding_resolves_to_workspace_root_for_create_agent
• test_session_metadata_round_trips_local_execution_binding
• test_session_metadata_defaults_missing_binding_to_local_from_repo_path
• test_cloud_workspace_binding_serializes_to_typed_dict
• test_binding_resolver_rejects_unknown_binding_kind
• test_run_agent_uses_resolved_workspace_root_from_binding
• test_restore_checkpoint_preserves_execution_binding
• test_session_owner_acquire_conflicts_with_live_lease
• test_session_owner_renew_rejects_stale_token
• test_run_agent_rejects_non_owner_instance
• test_restore_checkpoint_rejects_stale_owner
• test_close_session_rejects_stale_owner
• uv run pytest tests/ui/test_execution_binding.py tests/ui/test_session_owner_store.py tests/ui/test_session_manager_owner_checks.py tests/ui/test_session_manager_public_api.py tests/agentkit/storage/test_pg.py -v passes