feat(gptel-tools): wire git_status / git_log / git_diff as local tools

Three read-only git context tools so gptel can see what's changed
without me pasting `git status` / `git log` / `git diff` output into
every chat turn.  Builds the first batch from the ADOPT bucket in
`docs/design/gptel-tools-shortlist.org`.

Shape per tool:

- `gptel-tools/git_status.el` — `git status --short --branch` for a
  directory inside a git working tree under HOME.  Returns the
  porcelain output, or a "Clean working tree" marker when only the
  branch line is present.
- `gptel-tools/git_log.el` — `git log --oneline -nN` with an optional
  `--since` filter.  N defaults to 20, capped at 100; nil / non-
  integer / out-of-range N falls back to the default.
- `gptel-tools/git_diff.el` — `git diff [REF1 [REF2]] [-- FILE]`.
  Output capped at ~500KB so a runaway diff can't blow up context;
  truncation is reported inline.

Validation is uniform: path must resolve under HOME, must be a
directory, must be inside a git working tree (verified via
`git rev-parse --is-inside-work-tree`).  Color is disabled via
`-c color.ui=false` at the git level (`git status` doesn't accept
`--no-color` directly).

Tests run against real temp git repos created via `process-file`,
not mocked — there's nothing in gptel-tools/git_*.el that's
process-mockable in a meaningful way, and a real `git init` + a
couple of commits is cheaper than building a fake.  31 tests total:
7 for git_status, 11 for git_log, 13 for git_diff.

Wired into `cj/gptel-local-tool-features` so gptel exposes the
three tools on next restart.

1 files changed, 94 insertions, 0 deletions

diff --git a/gptel-tools/git_log.el b/gptel-tools/git_log.el
new file mode 100644
index 00000000..9cfae263
--- /dev/null
+++ b/gptel-tools/git_log.el
@@ -0,0 +1,94 @@
+;;; git_log.el --- Read-only git log tool for gptel -*- coding: utf-8; lexical-binding: t; -*-
+
+;; Author: Craig Jennings <c@cjennings.net>
+;; Keywords: convenience, tools, git
+
+;; This file is not part of GNU Emacs.
+
+;;; Commentary:
+
+;; Gptel tool returning `git log --oneline -n N' for a path under the
+;; user's home directory.  Read-only.  N is capped to keep the model's
+;; context budget predictable.
+
+;;; Code:
+
+(require 'gptel)
+
+(defconst cj/gptel-git-log--max-count 100
+  "Hard cap on the number of commits `git_log' will return.")
+
+(defconst cj/gptel-git-log--default-count 20
+  "Default commit count when the caller doesn't specify one.")
+
+(defun cj/gptel-git-log--validate-path (path)
+  "Validate PATH for a git log call.  Return the expanded path on success.
+Same contract as the git_status validator: must be under HOME, must
+be a directory, must be inside a git working tree."
+  (let ((full (expand-file-name (or path "~") "~")))
+    (unless (string-prefix-p (expand-file-name "~") full)
+      (error "Path must be within home directory: %s" path))
+    (unless (file-directory-p full)
+      (error "Not a directory: %s" full))
+    (let ((default-directory full))
+      (unless (zerop (process-file "git" nil nil nil
+                                   "rev-parse" "--is-inside-work-tree"))
+        (error "Not a git working tree: %s" full)))
+    full))
+
+(defun cj/gptel-git-log--effective-count (n)
+  "Return the commit count to use given caller-supplied N.
+Nil / non-integer N → `cj/gptel-git-log--default-count'.
+Values above `cj/gptel-git-log--max-count' get capped."
+  (cond
+   ((not (integerp n)) cj/gptel-git-log--default-count)
+   ((< n 1) cj/gptel-git-log--default-count)
+   ((> n cj/gptel-git-log--max-count) cj/gptel-git-log--max-count)
+   (t n)))
+
+(defun cj/gptel-git-log--run (path &optional n since)
+  "Run `git log --oneline -n N' in PATH.  Return the output as a string.
+SINCE, if a non-empty string, is passed as `--since=SINCE'."
+  (let* ((dir (cj/gptel-git-log--validate-path path))
+         (count (cj/gptel-git-log--effective-count n))
+         (args (list "-c" "color.ui=false"
+                     "log" "--oneline"
+                     (format "-n%d" count)))
+         (args (if (and (stringp since) (not (string-empty-p since)))
+                   (append args (list (format "--since=%s" since)))
+                 args))
+         (default-directory dir))
+    (with-temp-buffer
+      (let ((exit (apply #'process-file "git" nil t nil args)))
+        (unless (zerop exit)
+          (error "git log exited with %d: %s" exit (buffer-string)))
+        (let ((out (buffer-string)))
+          (if (string-empty-p out)
+              (format "No commits in %s matching the filter" dir)
+            out))))))
+
+(with-eval-after-load 'gptel
+  (gptel-make-tool
+   :name "git_log"
+   :function (lambda (path &optional n since)
+               (cj/gptel-git-log--run path n since))
+   :description "Return the output of `git log --oneline -n N' for a directory in the user's home tree.  Read-only.  N defaults to 20 and is capped at 100.  Use SINCE to filter commits more recent than a date expression git understands (e.g. '2 weeks ago', '2026-05-01')."
+   :args (list '(:name "path"
+                       :type string
+                       :description "Directory inside a git working tree.  Either an absolute path under the user's home directory or a path relative to it (e.g. 'code/myproject').")
+               '(:name "n"
+                       :type integer
+                       :description "Number of commits to return.  Defaults to 20; capped at 100."
+                       :optional t)
+               '(:name "since"
+                       :type string
+                       :description "Optional date expression for `git log --since='; e.g. '2 weeks ago' or '2026-05-01'."
+                       :optional t))
+   :category "git"
+   :confirm nil
+   :include t)
+
+  (add-to-list 'gptel-tools (gptel-get-tool '("git" "git_log"))))
+
+(provide 'git_log)
+;;; git_log.el ends here