Add AGENTS.md agent guidance

Co-Authored-By: Claude Fable 5 <[email protected]>

AGENTS.md

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+# AGENTS.md — bitblocks
+
+## Purpose
+
+The umbrella site for the BitBlocks stack (`bitblocks.network`): a static
+marketing/narrative page presenting the composable Nostr + Bitcoin building
+blocks (BitLogin, BitGate, BitFeed, BitUnlock, Tessera). Static site, no build
+step.
+
+## Repository map
+
+- `index.html` — the entire site, one page
+- `js/bitblocks.js` — the only script (theme toggle etc.), loaded with `defer`
+- `assets/` — `bitblocks.css` and self-hosted fonts
+- `docs/TODO.md` — backlog and design-review notes
+- `vercel.json` — deployment config: no build/install command, strict CSP headers
+
+## Build / test / validate
+
+There is no package.json, no build step, and no test suite. Validation is
+manual:
+
+- Run `python3 -m http.server 8080` from the repo root, then open
+  `localhost:8080` and check the page renders and the theme toggle works.
+
+There are no CI workflows in this repo.
+
+## Conventions
+
+- Plain hand-written HTML/CSS/JS; `js/bitblocks.js` is classic-script style
+  (IIFEs, no modules, no framework, no dependencies).
+- Theme-aware via CSS custom properties; JS only stamps the explicit
+  light/dark choice on the root element.
+- `vercel.json` enforces a strict CSP (`script-src 'self'`, no external
+  origins) — any change must keep working under that policy: no inline
+  scripts, no CDN resources.
+
+## Backlog
+
+The canonical backlog is `docs/TODO.md`.
+
+## Security
+
+- Static-site architecture is the standing assumption: no server code, no
+  secrets belong in this repo — never commit keys or tokens.
+- Treat any external content (fetched data, user input) as untrusted; today
+  the site fetches nothing, and the CSP is written to keep it that way.
+- Claim precision matters here (see `docs/TODO.md`): this page describes
+  security infrastructure, so copy changes should not overclaim.
+
+## Change policy
+
+- Keep diffs minimal and focused.
+- There are no tests to update; verify changes manually via the local server
+  before considering them done, and don't introduce changes that violate the
+  CSP in `vercel.json`.