refactor(ai-vterm): rename Claude-specific names to a generic "agent"

I may add other terminal agents to this launcher (aider, an open-source LLM TUI), so the buffer prefix, the user knob, and the internal helpers shouldn't say "Claude". The module name (ai-vterm) and the `cj/ai-vterm-*` customs were already generic. This finishes the job:

- buffer prefix `claude [<basename>]` -> `agent [<basename>]` (the `defconst` and the matching display-buffer-alist regex move together)
- `cj/ai-vterm-claude-command` -> `cj/ai-vterm-agent-command` (the default still runs the `claude` CLI, with a docstring note on swapping it)
- `cj/--ai-vterm-claude-buffers` / `-displayed-claude-window` / `-reuse-existing-claude` -> `-agent-*`, and their test files renamed to match
- prose in the module commentary and docstrings, plus the matching test docstrings and buffer-name literals

`vterm-config.el` hardcodes the same buffer prefix in `cj/--vterm-toggle-buffer-p` (F12 excludes agent buffers from its candidate set), so that literal moved too. Collapsing it into the shared `cj/--ai-vterm-name-prefix` is a cleanup for another day.

After a reload, a project's buffer opens as `agent [foo]` instead of `claude [foo]`. Old buffers keep their names until killed. I also corrected two stale `eshell-vterm-config.el` references in ai-vterm.el docstrings (that module was split into `vterm-config.el`).

Two things keep saying "Claude": the `cj/ai-vterm-agent-command` default value (the actual CLI), and the "Claude Code" example in `vterm-config.el`'s cursor-restore docstring (a concrete TUI example, not branding).

90 tests pass. `make validate-modules` clean.

1 files changed, 99 insertions, 0 deletions

diff --git a/tests/test-ai-vterm--reuse-existing-agent.el b/tests/test-ai-vterm--reuse-existing-agent.el
new file mode 100644
index 00000000..e6848014
--- /dev/null
+++ b/tests/test-ai-vterm--reuse-existing-agent.el
@@ -0,0 +1,99 @@
+;;; test-ai-vterm--reuse-existing-agent.el --- Tests for reuse-existing-agent action -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; The action looks for any window in the selected frame whose buffer
+;; satisfies `cj/--ai-vterm-buffer-p'.  When found, swaps that
+;; window's buffer for the one being displayed and returns the
+;; window.  When not found, returns nil so the next action in the
+;; chain runs.
+;;
+;; This is the action that keeps C-F9 (project-switch) from stealing
+;; a non-agent window when the user is focused inside agent.
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(add-to-list 'load-path (expand-file-name "tests" user-emacs-directory))
+(require 'ai-vterm)
+(require 'testutil-vterm-buffers)
+
+(ert-deftest test-ai-vterm--reuse-existing-agent-swaps-buffer-when-window-exists ()
+  "Normal: an agent window exists -> swap its buffer, return the window."
+  (cj/test--kill-agent-buffers)
+  (save-window-excursion
+    (delete-other-windows)
+    (let ((existing (get-buffer-create "agent [existing]"))
+          (new-buf (get-buffer-create "agent [new]"))
+          (split (split-window (selected-window) nil 'right)))
+      (unwind-protect
+          (progn
+            (set-window-buffer split existing)
+            (let ((result (cj/--ai-vterm-reuse-existing-agent new-buf nil)))
+              (should (eq result split))
+              (should (eq (window-buffer split) new-buf))))
+        (kill-buffer existing)
+        (kill-buffer new-buf)))))
+
+(ert-deftest test-ai-vterm--reuse-existing-agent-returns-nil-when-no-agent-window ()
+  "Boundary: no agent window in frame -> nil (chain continues to next action)."
+  (cj/test--kill-agent-buffers)
+  (save-window-excursion
+    (delete-other-windows)
+    (let ((new-buf (get-buffer-create "agent [no-existing]")))
+      (unwind-protect
+          (should (null (cj/--ai-vterm-reuse-existing-agent new-buf nil)))
+        (kill-buffer new-buf)))))
+
+(ert-deftest test-ai-vterm--reuse-existing-agent-leaves-non-agent-windows-alone ()
+  "Boundary: only non-agent windows in frame -> nil; other windows untouched."
+  (cj/test--kill-agent-buffers)
+  (save-window-excursion
+    (delete-other-windows)
+    (let ((code-buf (get-buffer-create "*test-code-buffer*"))
+          (new-agent (get-buffer-create "agent [new-here]"))
+          (other-win (split-window (selected-window) nil 'right)))
+      (unwind-protect
+          (progn
+            (set-window-buffer (selected-window) code-buf)
+            (set-window-buffer other-win code-buf)
+            (let ((result (cj/--ai-vterm-reuse-existing-agent
+                           new-agent nil)))
+              (should (null result))
+              (should (eq (window-buffer (selected-window)) code-buf))
+              (should (eq (window-buffer other-win) code-buf))))
+        (kill-buffer code-buf)
+        (kill-buffer new-agent)))))
+
+(ert-deftest test-ai-vterm--reuse-existing-agent-preserves-non-agent-window-when-swapping ()
+  "Normal: swap agent window only; the other window keeps its buffer.
+
+This is the C-F9-from-agent regression: with agent at the bottom
+and code on top, switching projects must replace the bottom window's
+buffer, not the top window's."
+  (cj/test--kill-agent-buffers)
+  (save-window-excursion
+    (delete-other-windows)
+    (let* ((code-buf (get-buffer-create "*test-code-top*"))
+           (agent-a (get-buffer-create "agent [a]"))
+           (agent-b (get-buffer-create "agent [b]"))
+           (top-win (selected-window))
+           (bottom-win (split-window top-win nil 'below)))
+      (unwind-protect
+          (progn
+            (set-window-buffer top-win code-buf)
+            (set-window-buffer bottom-win agent-a)
+            ;; Focus the agent window -- this is the regression scenario.
+            (select-window bottom-win)
+            (let ((result (cj/--ai-vterm-reuse-existing-agent
+                           agent-b nil)))
+              (should (eq result bottom-win))
+              (should (eq (window-buffer bottom-win) agent-b))
+              (should (eq (window-buffer top-win) code-buf))))
+        (kill-buffer code-buf)
+        (kill-buffer agent-a)
+        (kill-buffer agent-b)))))
+
+(provide 'test-ai-vterm--reuse-existing-agent)
+;;; test-ai-vterm--reuse-existing-agent.el ends here