Rework btw as an inline hidden thread

Move /btw off the old one-shot pager path and run it through a hidden forked thread so normal permissions, model settings, xhigh markers, and follow-up turns use the standard turn pipeline. Render the interaction in the bottom pane with wrapped output and explicit follow-up input, and remove the obsolete pager overlay entry point.

Verification: just fmt; cargo test -p codex-tui --lib btw; cargo test -p codex-tui --lib pager_overlay; cargo insta pending-snapshots; just fix -p codex-tui. Full codex-tui --lib still has unrelated status snapshot drift and fork_last stack overflow in this worktree.

Author: LEON-gittech

codex-cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@leonw24/open-codex",
-  "version": "0.129.17",
+  "version": "0.129.18",
   "license": "Apache-2.0",
   "bin": {
     "open-codex": "bin/codex.js"