Split Ambion package into application, host, protocol, and Node entry points

[andreisavu]

Motivation

• Reduce the default package surface to the small, application-facing vocabulary (the five primitives) and the minimal types required to use them.
• Surface advanced host concerns (runtimes, transports, persistence), wire protocol shapes, and Node filesystem adapters at explicit subpaths to make integration contracts clear and avoid leaking implementation details.
• Remove implementation classes and alias re-exports from the default entry point so hosts consume implementation-only APIs from explicit subpaths.

Description

• Shrunk the default entry point (packages/ambion/src/index.ts) to export only the application primitives (defineAgent, defineHuman, defineTool, defineWorkspace, startSession) and the narrow set of types needed to call them and consume their results.
• Added explicit entry modules packages/ambion/src/host.ts, packages/ambion/src/protocol.ts, and packages/ambion/src/node.ts, and wired package.json exports for ./host, ./protocol, and ./node so hosts and adapters import the correct surface.
• Replaced the publicly exported SeatActor class with a host-facing factory createSeatActor(...) and removed direct re-exports of Pi internals from the application default export, keeping those in src/internal.ts for tests and internal usage.
• Moved wire types/validators to the protocol entry point, runtime and storage adapters (including sqliteSessions) to the host entry point, and Node workspace adapters (memoryBackend, directoryBackend) to the node entry point.
• Updated the Cloudflare adapter and tests to import from the new explicit entry points and adjusted many tests to import internal-only helpers from the internal module where appropriate.
• Added a package API test (packages/ambion/test/package.test.ts) that asserts exact export lists for the default application entry point and the host, protocol, and node entry points to prevent API regressions.

Testing

• Ran TypeScript checks for the package: pnpm --filter @ambionframework/ambion run check:types — succeeded.
• Built the package: pnpm --filter @ambionframework/ambion run build — succeeded and emitted the new dist artifacts for the new entry points.
• Ran the Ambion test suite: pnpm --filter @ambionframework/ambion test — executed for the ambion package (no failures observed in this run).
• Attempted Cloudflare package typecheck: pnpm --filter @ambionframework/cloudflare run check:types — failed in this environment due to missing Cloudflare-specific type libraries and local package install; this is an environment/dependency issue rather than a change in the adapter code.

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