Harness spec 3.0.0-draft: prompts, the link, and the changelog

blocks/context.md tells the model about stubs and recall; blocks/workspace.md
tells a chat's model its view. blocks/library.md and blocks/prototypes.md are
the CLI's alone. acp.md: `instance` on session/new, a client-opened session
runs on the chat's model and no other (the own-model notice is gone),
capability `show` with `_lembas/show`, memory through the instance. README:
the new fields and the 3.0.0 section. VERSION 3.0.0-draft.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

harness/VERSION

@@ -1 +1 @@
-2.0.0
+3.0.0-draft