fix: read prompt and tools from the normalized transcript

pi 0.86.0 hands custom providers a TranscriptContext: `systemPrompt` and
`tools` are gone, and tool declarations live in the transcript's system
messages. Reading `context.tools` therefore sent zero tools to Devin, so
every Devin model answered that it cannot run commands, and the system
prompt was dropped with it.

Collapse the system messages first, then take the prompt and the current
tool set from the replayed transcript.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/stream.ts

@@ -4,9 +4,9 @@ import {
   type Api,
   type AssistantMessage,
   type AssistantMessageEventStream,
-  type Context,
   type Model,
   type SimpleStreamOptions,
+  type TranscriptContext,
   calculateCost,
   createAssistantMessageEventStream,
 } from "@earendil-works/pi-ai";
@@ -402,7 +402,7 @@ async function* streamChatEvents(args: {
 
 export function streamDevin(
   model: Model<Api>,
-  context: Context,
+  context: TranscriptContext,
   options?: SimpleStreamOptions,
 ): AssistantMessageEventStream {
   const stream = createAssistantMessageEventStream();