Merge remote-tracking branch 'origin/xtr/session-format-migration' into xtr/message-tool-call-id

# Conflicts:
#	.agents/notes/implemented/architecture/2026-06-20-branded-ids.i18n.yaml
#	.agents/notes/implemented/architecture/2026-06-20-branded-ids.md
#	.agents/notes/implemented/architecture/2026-06-20-branded-ids.zh.md
#	packages/fs/tool-fs/tests/read-image.spec.ts
#	packages/llm/llm-deepseek/tests/adapter.e2e.ts
#	packages/llm/llm-deepseek/tests/serialize.spec.ts
#	packages/llm/llm-pi-ai/src/context.ts
#	packages/llm/llm-pi-ai/tests/context.spec.ts
#	packages/llm/llm-pi-ai/tests/convert.spec.ts
#	packages/llm/llm/src/message.ts
#	packages/llm/llm/tests/content.spec.ts

docs/subsystems/llm-streaming.md

@@ -675,6 +675,8 @@ interface PreparedLlmCall {
   readonly retryPolicy: ResolvedRetryPolicy
   /** Detached context metadata resolved with the registration-bound call. */
   readonly context?: LlmModelContext
+  /** Exact model modalities captured with the adapter dispatch generation. */
+  readonly inputModalities?: readonly ModelModality[]
   /** Config fields materialized by the captured adapter rather than proposed by the caller. */
   readonly adapterDefaults: LlmCallConfigAdapterDefaults
   /**
@@ -730,6 +732,16 @@ declare abstract class LlmAdapter {
     model: string,
     _signal?: AbortSignal,
   ): Promise<LlmResolvedModelInfo>;
+  /**
+   * Bind exact model metadata and the eventual request dispatch to one adapter generation.
+   * Dynamic adapters override this so settings changes between preparation and
+   * dispatch cannot combine one generation's capabilities with another's endpoint.
+   * @param provider - registered provider route.
+   * @param model - exact model id.
+   * @param signal - cancellation for model resolution.
+   * @returns model metadata and a one-generation stream entry point.
+   */
+  async prepareCall(provider: string, model: string, signal?: AbortSignal): Promise<PreparedAdapterCall>;
   /**
    * Stream one model call as raw chunks. The only required method.
    * @param options - the fully-assembled request; implementations must honor `options.signal`.