feat(gptel-tools): harden path validation with file-truename realpath

Resolves PATH through file-truename before applying home-directory and
read/write checks across the path-handling tools (git_status, git_log,
git_diff, move_to_trash, read_text_file, update_text_file,
write_text_file, list_directory_files, read_buffer, web_fetch).
Without the resolve step, a symlink under HOME pointing outside HOME
would pass the prefix check but the tool would act on the real target
-- a symlink-escape.

move_to_trash also tightens the trash-bin construction (treats empty
file extensions correctly) and switches the "critical directories"
list to truename-resolved canonical forms so a symlinked ~/.config
can't be trashed via an aliased path.

update_text_file fixes an off-by-one in the line-count derivation
when the source content is empty.

Each source change pairs with tests in tests/test-gptel-tools-*.el
and tests/test-update-text-file.el covering the realpath escape
paths, the empty-extension trash case, and the empty-content line-
count edge.  Combined coverage is now 100% across all ten gptel-tools
source files: 516 / 516 executable lines, 217 tests.

1 files changed, 14 insertions, 0 deletions

diff --git a/tests/test-gptel-tools-read-buffer.el b/tests/test-gptel-tools-read-buffer.el
index 75efd604..0a854835 100644
--- a/tests/test-gptel-tools-read-buffer.el
+++ b/tests/test-gptel-tools-read-buffer.el
@@ -40,6 +40,14 @@
     (should (equal (cj/read-buffer--get-content (current-buffer))
                    "from buffer object"))))
 
+(ert-deftest test-gptel-tools-read-buffer-boundary-widened-content ()
+  "Boundary: returns the whole buffer even when the buffer is narrowed."
+  (with-temp-buffer
+    (insert "visible\nhidden\n")
+    (narrow-to-region (point-min) (line-end-position))
+    (should (equal (cj/read-buffer--get-content (current-buffer))
+                   "visible\nhidden\n"))))
+
 (ert-deftest test-gptel-tools-read-buffer-boundary-strips-text-properties ()
   "Boundary: the returned string has no text properties."
   (with-temp-buffer
@@ -56,5 +64,11 @@
   (should-error (cj/read-buffer--get-content
                  "test-gptel-tools-read-buffer-absent")))
 
+(ert-deftest test-gptel-tools-read-buffer-error-killed-buffer-object ()
+  "Error: a killed buffer object signals clearly."
+  (let ((buffer (generate-new-buffer "test-gptel-tools-read-buffer-killed")))
+    (kill-buffer buffer)
+    (should-error (cj/read-buffer--get-content buffer))))
+
 (provide 'test-gptel-tools-read-buffer)
 ;;; test-gptel-tools-read-buffer.el ends here