docs: correct the bot-score claims behind the browser poller

Every doc statement that explained a Cloudflare challenge as a "score"
was wrong. Researched against Cloudflare's own docs on 2026-08-12
(docs/research/cloudflare-bot-scoring-and-poll-cadence.md, 22 primary
pages plus RFC 9309): the 1-99 bot score is Enterprise Bot Management
only, free-plan zones get Bot Fight Mode signature matching and no score
at all, and no per-IP request rate is documented as an input to
challenge issuance. cf_clearance also expires in 30 minutes, so every
cadence at or above 1h re-solves the challenge regardless.

Docs only - no behaviour change. The 6h browser cooldown stays; its
justification is now cost (a serialized single-tab solve costs seconds,
a plain read costs one request), not a risk reduction nothing documents.

- AGENTS.md: the block is per-zone configuration plus request
  fingerprint, not IP reputation; comix.to turning its gate on
  2026-08-12 is the example. Residential egress avoids the
  cloud-hosting-IP signature rather than earning a better score. The UTC
  measurement stands but its mechanism is marked undocumented.
- backend/AGENTS.md: states why the browser cooldown is longer.
- ADR-0003, ADR-0006: dated corrections rather than rewrites. Both
  decisions stand on their other arguments (sweep depth, VPS memory).
- DEPLOY.md: a red smoke run means the Site's settings or this Chrome's
  fingerprint moved, not that "Cloudflare's scoring" did.

backend/AGENTS.md

@@ -149,7 +149,13 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   `LATEST_CHAPTER_POLL_ENABLED`/`_COOLDOWN`/`_BROWSER_COOLDOWN`/`_INTERVAL`/
   `_BATCH`/`_STAGGER` (background latest-chapter poller; defaults on,
   `1h` plain-TLS cooldown, `6h` browser cooldown, `10m`/`14`/`20s`; both
-  cooldowns have a `15m` floor).
+  cooldowns have a `15m` floor). The browser cooldown is longer for cost, not
+  for safety: a challenged page costs seconds of a serialized single-tab
+  browser, while a plain read costs one request. It buys no documented
+  reduction in challenge risk — free-plan zones have no bot score and no
+  published per-IP rate input, and `cf_clearance` expires in 30 minutes so
+  every cadence at or above 1h re-solves anyway —
+  `docs/research/cloudflare-bot-scoring-and-poll-cadence.md`.
   `USERSCRIPT_PATH` and `NOVEL_USERSCRIPT_PATH` (files served at
   `/u/{token}/manga-bookmark.user.js` and `/u/{token}/novel-bookmark.user.js`,
   defaults `/userscript/manga-bookmark.user.js` and