docs: add living architecture guidance

[zakoverflow]

Summary

• Add a repository-level ARCHITECTURE.md for note-c's core boundaries, data flow, public contracts, runtime model, and update triggers.
• Add docs/architecture/architecture.html as an interactive single-file visual architecture map with layer bands, dependency arrows, clickable nodes, view filters, source paths, contracts, and architecture hotspots.
• Add docs/architecture/architecture.json as a structured architecture map for future AI agents and tooling.
• Add docs/architecture/ maintenance rules, an initial ADR, and a reusable architecture template.
• Add AI tool entrypoints for Codex/OpenClaw, Claude, Gemini, Copilot, and Cursor so future agents read and update architecture docs alongside architecture-relevant changes.

Verification

• Validated docs/architecture/architecture.json with jq.
• Checked generated HTML for unresolved placeholders and obvious generated-token errors.
• Could not render a browser/canvas screenshot in this environment because no supported browser is installed and the canvas renderer reported node required.
• Documentation-only change; no build or unit tests run.