Provide a stable public API so dependents stop importing unplug.core.* #52

Server and MCP currently reach into SDK internals, which is fragile across refactors and minor bumps.

• unplug-server: unplug.core.policy, unplug.core.privacy, unplug.core.runtime.cache, unplug.ml.span_model; also mixes unplug.models with unplug.api.types, and tests import the deprecated unplug.core.cache.
• unplug-mcp: unplug.core.agent.boundaries for untrusted-content wrapping.

Proposal

• Expose stable, documented entry points (e.g. under unplug.api.*) for: boundary / untrusted-content wrapping, policy, privacy filtering, and the shared scan cache.
• Keep unplug.core.* private; add a thin compatibility shim where needed.
• Migrate server + mcp onto the public API and add an import-surface regression test.

Identified during a repository review.