feat(agent): multi-agent orchestration and session control

[Svagtlys]

Description

Multi-agent orchestration and session control: sub-agent spawning, turn arbitration, termination authority. Adds sessions.driver_participant_id (nullable, composite FK to session_participants, NULL = owner-driven), sessions.turn_policy (round_robin | llm_select | supervisor | handoff), optional turn cursor and budget columns, and the agent_run / agent_result / handoff / turn_assigned event kinds.

Why

The shipped transcript schema makes multi-party/A2A/autonomous sessions reachable without re-modeling, but no orchestration behavior exists. Termination authority and speaker selection are separate axes; a runaway-loop backstop (semantic + convergence + budget) is mandatory for any autonomous design.

Do not re-derive the reasoning: see the spec section "Design rationale for the deferred orchestration set" in .agents/specs/2026-09-13-vector-db-schema-orm-design.md.

Scope

• Additive Alembic revision: driver_participant_id, turn_policy, cursor/budget columns
• New EventKind values (TEXT-backed; no CHECK rebuild needed)
• Turn arbitration policy + runaway-loop backstop
• Tests

Blocked by #84. Spec: .agents/specs/2026-09-13-vector-db-schema-orm-design.md (Follow-up issues, item 3).