Adds docs/architecture/index.html — a polished, self-contained HTML
architecture reference with Mermaid diagrams (system/plugin model,
canonical state machine, end-to-end sequence, monorepo dependency map,
prompt assembly). Covers the 8 plugin slots, session lifecycle
(canonical vs legacy status), session manager + stale-runtime
reconciliation, flat-file storage, web data flow, prompt layering, and
cross-platform helpers. Every major claim cites real source as file:line.
Renders by opening the file directly (CDN Mermaid, no build step).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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