Restore input-window tint as an opt-in activity-phase hook

Commit e40d3ac applied the tint by calling --update-input-activity-indicator
directly from --set-activity-phase.  The rebase onto upstream kept upstream s
hook-based --set-activity-phase (pi-coding-agent-activity-phase-functions) and
dropped that call, so the tint silently stopped working.

Rather than re-patch core, provide a public opt-in handler matching the
documented hook convention.  pi-coding-agent-tint-input-window takes the
(CHAT INPUT OLD NEW REASON) signature and is enabled with:

  (add-hook (quote pi-coding-agent-activity-phase-functions)
            (function pi-coding-agent-tint-input-window))

--update-input-activity-indicator now takes the input buffer explicitly so
it composes with the hook (and is safe when the buffer is nil/dead during
teardown).  --set-activity-phase is left exactly as upstream wrote it.

Also pin package-user-dir to a project-local .cache/elpa in the Makefile so
test and dependency installs never pollute the developer s real
~/.emacs.d/elpa (which here is managed by straight.el).  Overridable via
make PACKAGE_USER_DIR=...

Author: Lugh

Makefile

@@ -1,7 +1,12 @@
 # pi-coding-agent Makefile
 
 EMACS ?= emacs
+# Keep package state project-local so test/dep installs never pollute the
+# developer's real ~/.emacs.d/elpa (which may be managed by straight.el etc.).
+# Override with `make test PACKAGE_USER_DIR=...` if needed.
+PACKAGE_USER_DIR ?= $(abspath .cache/elpa)
 BATCH = $(EMACS) --batch -Q -L . \
+	--eval "(setq package-user-dir \"$(PACKAGE_USER_DIR)\")" \
 	--eval "(add-to-list 'treesit-extra-load-path (expand-file-name \"~/.emacs.d/tree-sitter\"))"
 # Keep this checkout first in load-path even after package-initialize.
 LOCAL_LOAD_PATH = --eval "(setq load-path (cons (expand-file-name \".\") load-path))"

pi-coding-agent-ui.el

@@ -481,22 +481,37 @@ Blends the default background toward cornflower blue."
             (pi-coding-agent--blend-color bg tint amount))))
     (error nil)))
 
-(defun pi-coding-agent--update-input-activity-indicator (phase)
-  "Update the input buffer background tint for PHASE.
+(defun pi-coding-agent--update-input-activity-indicator (input-buf phase)
+  "Update INPUT-BUF background tint for PHASE.
 When PHASE is idle the background reverts to normal.  Uses
 `face-remap-set-base' so the tint covers the entire window
-including empty regions, not just text."
-  (when-let* ((input-buf pi-coding-agent--input-buffer))
-    (when (buffer-live-p input-buf)
-      (with-current-buffer input-buf
-        (if (equal phase "idle")
-            (when pi-coding-agent--input-activity-tinted
-              (face-remap-reset-base 'default)
-              (setq pi-coding-agent--input-activity-tinted nil))
-          (when-let* ((bg (pi-coding-agent--input-activity-bg)))
-            (face-remap-set-base 'default :background bg
-                                 :foreground (face-foreground 'default nil t))
-            (setq pi-coding-agent--input-activity-tinted t)))))))
+including empty regions, not just text.  Does nothing when INPUT-BUF
+is nil or dead, so it is safe to call during session teardown."
+  (when (buffer-live-p input-buf)
+    (with-current-buffer input-buf
+      (if (equal phase "idle")
+          (when pi-coding-agent--input-activity-tinted
+            (face-remap-reset-base 'default)
+            (setq pi-coding-agent--input-activity-tinted nil))
+        (when-let* ((bg (pi-coding-agent--input-activity-bg)))
+          (face-remap-set-base 'default :background bg
+                               :foreground (face-foreground 'default nil t))
+          (setq pi-coding-agent--input-activity-tinted t))))))
+
+;;;###autoload
+(defun pi-coding-agent-tint-input-window
+    (_chat-buffer input-buffer _old-phase new-phase _reason)
+  "Tint INPUT-BUFFER while the session is active in NEW-PHASE.
+Opt-in handler for `pi-coding-agent-activity-phase-functions'.  Tints the
+input window background whenever NEW-PHASE is anything other than \"idle\"
+and removes the tint when it returns to \"idle\".
+
+The remaining arguments are accepted to match the abnormal-hook calling
+convention and are ignored.  Enable with:
+
+  (add-hook \='pi-coding-agent-activity-phase-functions
+            #\='pi-coding-agent-tint-input-window)"
+  (pi-coding-agent--update-input-activity-indicator input-buffer new-phase))
 
 ;;;; Language Detection