Export the dev-runtime provider protocol types (start context, session, inspection envelope, generation store, MCP registry) from agent-bundle/api #485

Problem

dev.runtime.provider is a documented configuration key (website/docs/en/reference/configuration.mdx:153) and agent-bundle/api exports the provider entry type — DevRuntimeProvider, CreateDevRuntimeProvider, AgentBundleDevRuntimeConfig (packages/agent-bundle/src/api.ts:245-249). But a provider cannot be written against that export alone: every type the start() contract hands in or expects back — the start context, the session, the inspection envelope, the MCP server descriptor, the generation store and MCP registry a session must drive — lives only in packages/agent-bundle/src/dev/**. The one example that implements a provider therefore reaches into the package source with ../../../../packages/agent-bundle/src/dev/* imports, which contradicts the "public exports only" rule for examples/* and means no out-of-tree consumer can implement dev.runtime.provider with types at all.

This is distinct from #459 (what a context provider receives at request time); this issue is about the dev-runtime provider protocol behind dev.runtime.provider.

Evidence

All import sites are in examples/rsc-agent-runtime on main (after #473 moved JsonObject/JsonValue to the root export and DevRuntimeProvider to agent-bundle/api; the rest have no public home):

The example's README already labels itself "an architecture experiment, not an agent-bundle public API", so it does not block anything today; it does mean the dev.runtime.provider key is documented without a typed way to satisfy it.

Proposed shape

Export the provider protocol from agent-bundle/api, next to DevRuntimeProvider:

1. Types the DevRuntimeProvider contract references transitively (everything a provider must accept or return): the runtime-provider.ts and runtime-protocol.ts rows above, as export type. These are already the wire/session protocol the Workbench consumes, so exporting them is a naming decision, not a new surface.
2. The two error classes DevRuntimeUnavailableError and DevRuntimeGenerationConflictError as values — a provider has to throw them to get the documented Workbench behaviour.
3. RuntimeGenerationStore and RuntimeMcpRegistry as values plus their option/candidate types. If keeping the classes internal is preferred, DevRuntimeStartContext should instead hand a provider pre-constructed store/registry instances so a provider never needs to import them.
4. Leave ProjectService, ArtifactService, EpochStore, resolveDevRuntimeProvider, digest, stableJson internal; they are reached only from the example's tests, which can move to the agent-bundle/test dev-epoch level (runDevEpochContractMatrix) or stay deep-import as test-only wiring.

TypeDoc compiles packages/agent-bundle/src directly, so the api reference page follows automatically; the pnpm docs:site:build gate applies.

Acceptance

• import type { DevRuntimeStartContext, DevRuntimeSession, DevRuntimeInspectionEnvelope, DevRuntimeMcpServerDescriptor, /* … */ } from 'agent-bundle/api' resolves, and import { DevRuntimeUnavailableError, DevRuntimeGenerationConflictError } from 'agent-bundle/api' resolves.
• examples/rsc-agent-runtime/src/** contains no packages/agent-bundle/src path import (rg "packages/agent-bundle/src" examples/rsc-agent-runtime/src is empty); the four src/dev/*.ts and src/runtime/contracts.ts files import from agent-bundle/api instead.
• The public-API declaration test (packages/agent-bundle/tests/public-api.test.ts / public-api-packed.test.ts) covers the new names, and website/docs/{en,zh}/reference/configuration.mdx dev.runtime.provider links to the agent-bundle/api types a provider implements.
• A changeset (patch) names the new agent-bundle/api exports.

Found while auditing the examples against the public surface in #473.