[P1] Extract flagship experiences from the public control-plane core

[0YHR0]

Problem

AgentMesh still mixes control-plane code with flagship experiences. Music Studio is behind a
Runtime Extension boundary, but its implementation and assets remain in the public core repository;
Office and Virtual Company concerns also make the required adoption surface look larger than it is.

Proposal

Enforce a one-way boundary:

Experience repositories → public AgentMesh SDK/API
AgentMesh core ✕ experience implementation imports

Move Music Studio into its own private extension repository first. Keep only generic Company,
Task, Artifact, governance, extension, and UI integration contracts in core. Treat Office as an
optional visualization package that consumes public APIs.

Acceptance criteria

• Core boots, runs minimal E2E, and builds its wheel without Music Studio/Office implementation or
  assets present.
• Music Studio installs from its private locked wheel and retains current public routes through the
  extension registrar.
• No core module imports a vertical experience namespace; CI enforces dependency direction.
• Extension uninstall/disable leaves no broken navigation or startup dependency.
• Public README clearly separates Core, optional Experiences, and private/commercial extensions.
• Private extension CI tests against the oldest and newest supported core API versions.
• Upgrade and rollback preserve persisted scenario objects or explicitly block incompatible change.

Non-goals

• Removing generic Company/Organization primitives required by multiple experiences.
• Open-sourcing private scenario implementations.
• Designing extension-owned arbitrary database migrations before isolation/version policy exists.

Dependencies

Uses the existing Runtime Extension lock and should align with Managed Agent Runtime/Capability
contracts rather than inventing another plugin mechanism.