refactor(runtime): collapse speculative portability layers

Remove the one-consumer bounded-read primitive and shared terminal lifecycle controller, make terminal cleanup one awaited provider operation, and reuse one Code Runtime contract suite. Keep only reproduced cancellation and policy fixes; defer unproven replacement, prompt-attribution, and streaming-frame concerns to scoped markers.

packages/skill/skill-local/tests/skill-local.spec.ts

@@ -92,12 +92,6 @@ class TestFileSystem extends FileSystem {
     return text
   }
 
-  override async readTextBounded(target: FsTarget, maxBytes: number, signal?: AbortSignal): Promise<string> {
-    const text = await this.readText(target, signal)
-    if (Buffer.byteLength(text) > maxBytes) throw new Error('too large')
-    return text
-  }
-
   override async streamText(_target: FsTarget): Promise<AsyncIterable<string>> {
     throw new Error('not needed in skill tests')
   }