docs(orchestrator): codify event ownership, audit transcript, routing, and lifecycle contracts

[AiRanthem]

Summary

• Align pkg/orchestrator design and plan docs to remove implementation ambiguity.
• Define strict event ownership: engine owns task/session/route events, invoker owns request/transcript/chunk events.
• Enforce audit-first then bus event emission order globally.
• Require audit transcript to store fully rendered invoke request (Task/Session/Request IDs, runtime, per-part role, attachments metadata, diagnostics, registry snapshot/version) with redaction policy only.
• Clarify Submit(ctx) and Wait(ctx) lifetime semantics so submit Waiters cannot cancel background task.
• Enforce root no-Salutation routing: strict explicit default workspace, otherwise fail fast with registry.ErrNoDefaultWorkspace.
• Set upstream migration order before root engine integration and mandate fake agentio.Agent for engine tests until contracts complete.

Scope

• Updated files:
  • pkg/orchestrator/DESIGN.md
  • pkg/orchestrator/audit/PLAN.md
  • pkg/orchestrator/events/PLAN.md
  • pkg/orchestrator/invoker/PLAN.md
  • pkg/orchestrator/prompt/PLAN.md
  • pkg/orchestrator/protocol/PLAN.md
  • pkg/orchestrator/registry/PLAN.md
  • pkg/orchestrator/session/PLAN.md
  • pkg/orchestrator/task/PLAN.md

Notes

• No Go code changes.
• No changes to pkg/agentio or pkg/bridge.