docs: sync AGENTS.md to current architecture (internal/, Cinder, confirm-gated, edge-tab)

Captures what shipped on the branch:
- backend split into internal/ packages; composition root = main.go
- web UI go:embed now lives under internal/web/; Dockerfile must copy tree
- impeccable detector caveat (root-absolute /static/ paths) and false-clean
- confirm-row pattern for archive/finish/remove; --ember reserved
- edge-tab hitbox design (7x44 visible, 28x72 hit, touch-action + arm hold)
- Cinder design system section + ember-law reference

AGENTS.md

@@ -27,17 +27,43 @@ Bromite userscript (isolated world, per-site adapters, localStorage cache)
 ```
 
 - **Backend** (`backend/`): stdlib `net/http` (handful of routes, no framework) + `modernc.org/sqlite` (pure Go, `CGO_ENABLED=0` -> static binary -> distroless/scratch image). Reverse proxy terminates TLS; Go service listens plain `:8080`.
+  Single binary, split into packages under `backend/internal/`: `store`
+  (Bookmark type, SQLite persistence, migrations), `latest` (background
+  poller, site parsers, TLS fetcher), `session` (cookie signing, login
+  rate limiter), `httpmw` (Auth/Gzip/CORS middleware), `api` (JSON
+  bookmark handlers), `userscript` (userscript-serving handler), `web`
+  (browser UI handler + `templates/` + `static/`, `go:embed`-ed).
+  `backend/main.go` is the composition root — the only place that wires
+  packages together into `newRouter`. Root-level `*_test.go` hold
+  integration tests that exercise the full router; unit tests for a
+  package live beside it under `internal/`.
 - **Single-user store.** One `bookmarks` table keyed `<site>:<series_id>` (`asura`|`demonic`). Sync **last-write-wins**. Schema + endpoint list in plan.
 - **Endpoints:** `GET /bookmarks`, `PUT /bookmarks/{key}` (upsert; see `updated_at` rule below), `DELETE /bookmarks/{key}`, `GET /healthz` (no auth).
 - **Web UI:** same binary serves password-gated browser UI on second
   hostname — `GET /` (list, or login page when no session),
   `POST /login`, `POST /logout`, `GET /static/*`, htmx fragment endpoints
-  under `/ui/*`. Templates/assets `go:embed`-ed, so `backend/Dockerfile`
-  must copy `templates/` and `static/` plus `*.go`. Sessions = stateless
+  under `/ui/*`. Templates + assets `go:embed`-ed under
+  `backend/internal/web/`, so `backend/Dockerfile` must copy the whole
+  `internal/` tree, not just `*.go`. Sessions = stateless
   HMAC cookies keyed off `API_TOKEN`; `WEB_PASSWORD` gates them, when empty
   web routes not registered at all. UI mutations read-modify-write
   through `Store.Get` + `Store.Upsert` so `updated_at` rule stays one
   place. See `docs/superpowers/specs/2026-07-25-web-ui-design.md`.
+  **Design-tool caveat:** templates link `/static/style.css` root-absolutely
+  (correct — served from `/`), but impeccable detector resolves
+  stylesheet href with `path.resolve(fileDir, href)`, drops directory
+  on leading `/` and silently skips file. Relative hrefs don't help
+  either: template's directory isn't its served path. So
+  `detect.mjs backend/internal/web/templates` reports **false clean** —
+  always pass `backend/internal/web/static` too. One finding there,
+  `overused-font` on "Instrument Serif", deliberate identity choice, not debt.
+- **Every action that moves series out of list is confirm-gated.**
+  Archive, finish, remove each open own `.confirm-row` disclosure
+  (`toggleConfirmRow(key, kind)` in `filter.js`, `kind` ∈
+  `archive|finish|remove`); restore fires instantly since it's the reversal.
+  Remove's row wears ember wash, two reversible ones wear `.calm` grey.
+  `--ember` stays reserved for new-chapter signal: busy bar and inline
+  error use `--mute`.
 - **Latest-chapter poller:** ticker goroutine in same binary re-checks
   each bookmarked series' newest published chapter from backend's own
   network access, so `latest_chapter` stays fresh when user not
@@ -96,7 +122,14 @@ Bromite userscript (isolated world, per-site adapters, localStorage cache)
 5. **UI** — rendered inside **Shadow DOM** root to isolate from site CSS
    (critical on mobile). Three tabs (All / Favourites / Archived) + row of
    link chips to web UI and both manga sites; `WEB_BASE` sits in CONFIG
-   block next to `API_BASE`.
+   block next to `API_BASE`. FAB is `7 × 44` edge tab whose *hit* area
+   widened to `28 × 72` by invisible `#hit` child; `#fab` must keep
+   `touch-action: none` and must **not** regain `overflow: hidden`. Since
+   `touch-action` resolved at gesture start, strip can't be both
+   browser-scrolled and script-dragged, so `makeDraggable` splits by intent: swipe
+   from `#hit` scrolls via `window.scrollBy`, hold of `ARM_MS` arms
+   reposition drag, visible sliver drags with no hold. See
+   `docs/superpowers/specs/2026-07-28-edge-tab-hitbox-design.md`.
 6. **SPA navigation** — Asura is Astro, client-routed on comic/chapter pages: patch `history.pushState`/`replaceState` + listen `popstate`, re-run `detect()` on URL change so auto-update fires w/o reload. Demonic uses classic reloads (initial `document-idle` run suffices).
 
 ### Live URL shapes (verified 2026-07-26, may drift — re-check against live pages before trusting)
@@ -135,15 +168,55 @@ Smoke test: `curl` endpoints w/ `Authorization: Bearer <token>`; confirm `OPTION
 
 `tea` prints output as rendered boxes not plain text; PR URL lands on last line.
 
+## Design system
+
+Web UI + userscript panel follow **Cinder**, rules in `docs/design-system.md`
+— source of truth Claude Design project `mangaBookmark Web UI`
+(`969ac210-fe02-4c01-ae1b-9a271dcc779a`). Read it before touching
+`backend/internal/web/static/style.css`, `backend/internal/web/templates/*`, or userscript
+`TEMPLATE`/`CSS`. Core law: **ember means new chapter only** — no other
+state (busy, error, destruction) may use `--ember`; destruction gets
+`--danger`. No cards/corners/shadows, one `--measure: 760px` column, tokens
+only (never hardcode hex outside `:root`), both colour branches touched
+together. Any move that pulls series out of list (archive/finish/remove)
+must be confirm-gated via its own `.confirm-row`; only restore fires
+instantly.
+
 ## Security invariants
 
 - Auth on `/bookmarks*`: require `Authorization: Bearer <API_TOKEN>`, **constant-time compare**, 401 otherwise.
 - CORS: reflect `Origin` only when in `ALLOWED_ORIGINS`; allow `GET,PUT,DELETE,OPTIONS` + headers `Authorization,Content-Type`; answer preflight `OPTIONS` w/ `204`.
 
+## Comments
+
+Comment only if code alone can't carry info. Cost per read — must earn spot.
+
+Write for:
+- Why not what. Tradeoffs, non-obvious decisions.
+- Load-bearing detail looking incidental — say so if "simplify" breaks it.
+- Non-local consequence, invisible from function alone.
+- Wire format / encoding / interface contract — save callers re-deriving.
+- Gotcha/workaround, with ref if exists.
+- Domain/business rule not derivable from code.
+
+Skip:
+- Restating code (no `// increment i` above `i++`).
+- Trivial getter/setter/pass-through.
+- Banners, dividers, `// helpers`.
+- Change narration (`// fix bug`, `// as requested`, `// new impl`) — git's job.
+- Commented-out code — delete.
+- TODO without concrete action.
+
+Style: one dense comment over function beats one per line inside. Tight, no worked example unless bug subtle. Wrong comment worse than none — update/delete on change. Default fewer — sparse+high-signal beats comprehensive.
+
+Test: "competent reader get this from code in few sec?" Yes → skip. Needs detour through another file/spec/git-blame → write it.
+
 ## Relevant skills
 
 `multi-stage-dockerfile` and `docker-compose-orchestration` for container work (referenced in plan).
 
+`golang-code-style`, `golang-error-handling`, `golang-performance`, `golang-testing` for backend Go work.
+
 ## graphify
 
 Project has knowledge graph at graphify-out/ w/ god nodes, community structure, cross-file relationships.