feat(x/core): real bridges + DI assembly for the turn runtime (stage 4)

Five bridges adapt existing app code to the runtime seams, each unit-
tested against injected fakes:

- RealAgentResolver: loadAgent + dynamic builders -> immutable
  ResolvedAgent. Model precedence override > agent > app default; tool
  descriptors from BuiltinTools (zod -> JSON schema) and MCP
  attachments (toolId schemes builtin:/mcp:server:tool); ask-human as
  the async requiresHuman tool. System prompt composed via
  composeSystemInstructions — extracted verbatim from streamAgent so
  old and new runtimes share one implementation — driven by the new
  opaque RequestedAgent.overrides.composition (session-sticky inputs
  preserve provider prefix caching; per-message context stays on
  UserMessage.userMessageContext as today).
- RealModelRegistry: models.json -> live AI SDK model; one streamText
  step (stopWhen: stepCountIs(1)) normalized into deltas, durable step
  events, and the completed assistant message.
- RealToolRegistry: execTool dispatch (builtins + MCP) with per-call
  abortRegistry bracketing and tool-output-stream -> durable progress.
- RealPermissionChecker: getToolPermissionMetadata rules (command
  allowlist, workspace file boundaries); session grants deferred.
- RealPermissionClassifier: classifyToolPermissions with conversation
  context now threaded through the classifier batch.

Interface refinements (doc updated): ToolExecutionContext gains
turnId/toolCallId; classifier takes a batch with turnId + messages;
TurnRuntime dep renamed bus -> lifecycleBus and SessionsImpl bus ->
sessionBus for strict-PROXY DI. Container registers the whole stack
(FS repos rooted at WorkDir/storage); the legacy agentRuntime
registration became lazy to survive the new import-order cycle.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/x/packages/shared/src/turns.ts

@@ -32,6 +32,12 @@ export const RequestedAgent = z.object({
     overrides: z
         .object({
             model: ModelDescriptor.optional(),
+            // Opaque composition hints interpreted by the agent resolver
+            // (e.g. work-dir id, voice/search/code modes). Persisted verbatim
+            // for audit. The resolver decides which keys affect the system
+            // prompt; keeping prompt-affecting inputs session-sticky is what
+            // preserves provider prefix caching across turns.
+            composition: z.json().optional(),
         })
         .optional(),
 });