docs(agents): rendering-ownership rule — templ renders, JS decides what shows

Codifies the standing rule discussed for the templ/JS split: content
derived from server data is rendered by templ and refreshed via templ
partials (htmx.ajax), pure client-local state may render in TS/Alpine,
web/src is TypeScript-only, and inline <script> in templ files requires
a pre-paint justification (theme_flash_guard.templ only). Lists the
sanctioned JS islands (reader, Chart.js, toasts, password validation).

Also refreshes gotcha #1: the 'harmlessly masked' aside is stale — the
quoted-attribute sweep fixed every occurrence; the note now points at
templates.JSONArg for free text.

AGENTS.md

@@ -4,6 +4,34 @@ Standing rules for any agent or developer working in this repo.
 Project-wide context lives in the sibling repos' AGENTS.md files
 (../bookhoard-app/AGENTS.md is the Android client's).
 
+## Rendering ownership (templ vs TypeScript) — ALWAYS apply
+
+The point of templ + htmx + Alpine is: the server renders the UI, and
+JavaScript is kept minimal. The rule that keeps it that way:
+
+> **JavaScript decides what shows. templ decides what it looks like.**
+
+- Content derived from **server data** (DB rows, search results, match
+  lists, conflict cards, any list/card/badge/table) is rendered by
+  templ. When it changes client-side, fetch a **templ partial** from an
+  SSR route and swap it in (`htmx.ajax(...)`), never rebuild it from a
+  JSON payload with `innerHTML` string templates.
+- **Pure functions of client-local state** (open/closed panels, tabs,
+  password strength from a typed value, toasts) may render in
+  TS/Alpine — no server round trip needed.
+- `web/src` is **TypeScript only** — no new `.js` source files.
+  Vendored third-party and vite build output under `web/static` are
+  exempt.
+- Inline `<script>` in a `.templ` file needs a reason a bundled module
+  cannot do the job (pre-paint execution). Currently sanctioned:
+  `theme_flash_guard.templ` only.
+
+Sanctioned JS islands (engines, not UI renderers — exempt from the
+above): the reader (foliate-js + `web/src/reader/`), Chart.js graphs in
+stats, toasts, password validation. Anything else that renders app UI
+in TypeScript is a defect; if a templ twin of it exists, one of the two
+must be deleted.
+
 ## Library visibility (family-lens rule) — ALWAYS apply
 
 Per-user library visibility (`library_visibility` table, default-ALLOW:
@@ -96,9 +124,11 @@ of a server bug: stop and flag the user.
    values** — it emits the literal text. `x-init="f('{ col.ID }')"`
    shipped a request to `/api/collections/{ collection.ID }` found by
    the request logs; the whole attribute value must BE an expression:
-   `x-init={ "f('" + col.ID + "')" }`. Note
+   `x-init={ "f('" + col.ID + "')" }`. (Every occurrence of this bug —
-   `class="theme-{ user.Theme }"` has always been literal (harmlessly
+   including `class="theme-{ user.Theme }"` on 22 page bodies and 26
-   masked by the JS theme engine).
+   attribute interpolations — was swept and fixed in 2026-10; when
+   adding attributes with dynamic values, the whole value is an
+   expression, and free text goes through `templates.JSONArg`.)
 2. **SettingsRegistry caches**: writes must go through the settings
    API (Reload on save). Direct SQL writes to system_settings leave
    the cache stale until the next API-path write — never "preview"