feat(signal): dock chat buffer to bottom 30% and add cancel binding

I added a display-buffer-alist entry matching "*Signel:" chat buffers and routing them through display-buffer-at-bottom with window-height 0.3. The signel fork's signel-chat now uses pop-to-buffer instead of switch-to-buffer, which is what makes the rule apply. Without that switch the buffer replaces the current window and skips display-buffer entirely.

Two new tests in test-signal-config.el lock the entry shape and the regex's buffer-name match set. A new test-signel-cancel-input.el covers the fork's C-c C-k handler. It clears the editable region between signel--input-marker and point-max, then quit-windows so the buffer survives the dismiss.

Closes the "Chat buffer placement + exit keys" task filed during the 2026-05-28 manual-verify walk.

1 files changed, 10 insertions, 0 deletions

diff --git a/modules/signal-config.el b/modules/signal-config.el
index 00279d4c..102ece86 100644
--- a/modules/signal-config.el
+++ b/modules/signal-config.el
@@ -127,6 +127,16 @@ time."
   (when (file-readable-p cj/signal-private-config-file)
     (load cj/signal-private-config-file nil t)))
 
+;; Chat buffers (named `*Signel: <id>*') open in the bottom 30% of the
+;; frame rather than wherever display-buffer's fallback rule picks.
+;; The fork's `signel-chat' uses `pop-to-buffer', so this entry applies.
+(add-to-list
+ 'display-buffer-alist
+ '("\\`\\*Signel: "
+   (display-buffer-reuse-window display-buffer-at-bottom)
+   (window-height . 0.3)
+   (reusable-frames . nil)))
+
 ;;; Connection guard, contact fetch, and cache
 
 ;; Forward declarations: signel.el is loaded by the use-package above (with