Add HLD/LLD architecture page and clarify agent-framework vs story-tree branching
Adds docs/architecture.html (a standalone walkthrough of the context budget, world-state engine, story tree, egress fix, and pen-test findings) and links it from the landing page. Also clarifies in GUIDE.md §1.8 that "no branching" describes per-turn control flow, not the story tree's branching data.
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