fix: correct the pi wheel docs and comments (release review)

The gate comment in terminal-ui.js said a stale-false flag falls through
to the PageUp/PageDown fallback, which is false for pi since the second
commit of this PR removed that fallback. The FOOTGUN comment above it made
the same claim. Say what happens instead.

"Every pi below 1.0.0 emits no mouse enables" was too absolute: regular
was the default there, but fullscreen exists from 0.84.0. Measured with
--tui-mode fullscreen under tmux on 0.84.1, 0.85.0, 0.86.0 and 0.99.2:
alternate_on=1, mouse_any_flag=1, mouse_sgr_flag=1. Default settings on
0.99.2 report all 0.

Document that Wheel Scrolls Local History leaves the wheel dead in
fullscreen pi (wiki, integration guide, changeset), add a pi block to
Troubleshooting, cross-reference the accepted exception from the dead
gesture rule in architecture-invariants, and mark the pi-integration-plan
passages this PR reversed as superseded.

Comments and docs only, no runtime change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/config/cli-registry/stock.ts

@@ -1186,8 +1186,10 @@ const PI: CliEntry = {
     // empty local buffer and went dead until a redraw. Same shape as copilot: strip the DECSETs
     // (a drag selects locally), publish `cliMouseTracking`, and forward the wheel by hand from
     // that flag. pi scrolls on SGR wheel reports, one row each at a low rate, faster on a spin.
-    // `tuiMode: regular` (and every pi below 1.0.0) is inline and emits no mouse DECSET, so the
-    // strip finds nothing and the flag stays false.
+    // `tuiMode: regular`, the default below 1.0.0, is inline and emits no mouse DECSET, so the
+    // strip finds nothing and the flag stays false. A pi from 0.84.0 set to fullscreen does emit
+    // them (0.84.1 and 0.99.2 measured: alternate_on=1, mouse_any_flag=1, SGR) and the flag
+    // covers it.
     // ⚠️ No `transcriptPageKeys`, though fullscreen pi does page on PageUp/PageDown. pi binds
     // those keys to its editor in regular mode too, where they page nothing and move the caret
     // of a multi-line draft, so a wheel over a fresh regular pane sent the next keystroke to the