feat(agent): design agent lifecycle model

[Svagtlys]

Closes #25

Description

Implement the agent definition/instance split: definitions (agents) are persistent config with a definition-level active|paused gate, a model_binding and assignments; instances (agent_instances) are ephemeral rows binding one definition to one session, existing only while they can take turns — hard-deleted on destroy, carrying no context (the session transcript and the vault own all context).

Implementation

• octave.db.types: AgentStatus, InstanceStatus, TagModelBinding/ExplicitModelBinding/ModelBinding (discriminated union), AgentAssignments
• agents rework: model_binding JSON replaces model_tag (data migration copies old tags into tag-form bindings), new assignments JSON
• agent_instances table: unique agent×session binding, RESTRICT on definition delete, CASCADE on session delete, idle default
• upgrade(sync_url, revision=) supports seeding at older schema revisions (pins the model_tag data migration in tests)
• AgentInstanceManager (octave.agent.instances) — single write path, VaultStore convention (caller-supplied AsyncSession, never commits):
  • spawn: gates on paused definitions, terminal sessions, duplicates, missing/malformed bindings; ensures Participant + membership (re-invite resets left_at)
  • begin_turn/end_turn: atomic idle→active turn mutex; pause re-check at turn start
  • destroy: plain delete (archival decoupled to the turn boundary); reconcile: resets stale active rows after unclean shutdown
  • resolve_model: fail-loud binding resolution with injected tag_lookup (tag→model lookup lands with Inference feat(ui): create responsive layout components and shared UI library #7)
• Public surface exported from octave.agent; SDK quarantine (no openai/mcp imports) intact

References

• Design doc: .agents/specs/2026-09-27-agent-lifecycle-model-design.md
• Implementation plan: .agents/specs/2026-09-27-agent-lifecycle-model.md

Checklist

• Tests passing (356 passed)
• Lint clean (ruff + mypy strict)
• Frontend builds (not applicable — no frontend changes)