refactor(ambion): split application surface from host and protocol exports

[andreisavu]

Motivation

• Make the package root a small, well-documented application surface focused on agents, humans, tools, workspaces, sessions, visits, messages and session events so application authors import only what they need.
• Surface host integration points (Runtime, Transport, Clock, session repo helpers, workspace backends) separately for embedding environments to implement host-specific concerns.
• Expose the advanced, JSON-safe seat protocol separately for transport authors so only the transport implementation depends on low-level wire shapes and seat actor types.

Description

• Reduced packages/ambion/src/index.ts to the application-facing API and moved host integration exports into a new packages/ambion/src/host.ts module that re-exports runtime, session repo and workspace backend types and helpers.
• Added packages/ambion/src/protocol.ts to export SeatActor, inProcessTransport, wire assertion helpers and the JSON-safe types (Commit, LeaseRow, ViewResponse, Wake, etc.) needed by a transport implementation.
• Updated packages/ambion/tsdown.config.ts and packages/ambion/package.json to build and publish explicit subpath exports for . (application), ./host, and ./protocol.
• Migrated internal tests to import from the appropriate surface (root, /host or /protocol), added lightweight compile fixtures in packages/ambion/test/imports/*, and revised README.md and packages/ambion/README.md so the first example uses only the root package.

Testing

• Built the package with pnpm --filter @ambionframework/ambion run build using Node.js 22.19.0 and tsdown, and the build completed successfully.
• Type-checked the package with tsc --noEmit -p packages/ambion/tsconfig.json (Node.js 22.19.0) with no type errors after updating test imports.
• Ran the test suite with pnpm --filter @ambionframework/ambion test (Node.js 22.19.0), which executed 20 test files and passed 371 tests.
• Ran git diff --check to ensure no whitespace/punct issues; attempted lint with biome but the tool was not available in the runtime so the lint step was not executed.

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Codex Task