refactor(integrations): five hygiene fixes from the module-by-module re-review

- markdown-config.el: two related fixes on `markdown-preview'.
  First, the URL was `https://localhost:8080/imp' but simple-httpd
  serves plaintext on port 8080 -- the browser hit a TLS handshake
  against a non-TLS listener and the preview never rendered.  Changed
  to `http://' and switched from `browse-url-generic' to plain
  `browse-url' so the user's default protocol handler picks the
  browser.  Second, the function used to start the network listener
  as a side effect of opening a preview; that's split into a
  separate `cj/markdown-preview-server-start' command and
  `markdown-preview' now signals a `user-error' (with the recovery
  command in the message) when the server isn't running.

- slack-config.el: wrap the
  `which-key-add-keymap-based-replacements' call in
  `with-eval-after-load 'which-key'.  Matches the pattern other
  config modules use and means a slow / missing which-key load
  won't block requiring slack-config.

- ai-vterm.el: pass the inner shell-command-string through
  `shell-quote-argument' before wrapping in the tmux invocation.
  The default value with embedded double quotes was safe under the
  prior literal-single-quote wrap, but a user-customized
  `cj/ai-vterm-agent-command' containing a single quote silently
  broke the shell parse.  Two existing tests updated to tolerate
  the post-quote escape shape; new regression test asserts a
  single-quote-bearing custom command survives.

- eshell-config.el: scope the `TERM=xterm-256color' override to
  eshell-spawned processes only via an `eshell-mode' hook that
  prepends to a buffer-local `process-environment'.  The previous
  global `setenv' at config-time changed `TERM' for every
  subsequent `start-process' across the Emacs session, so any
  subprocess (not just eshell pipelines) inherited
  `xterm-256color' regardless of whether the receiver could
  interpret the escapes.

1 files changed, 11 insertions, 2 deletions

diff --git a/modules/eshell-config.el b/modules/eshell-config.el
index 62b3a7ab..4d180d1c 100644
--- a/modules/eshell-config.el
+++ b/modules/eshell-config.el
@@ -128,8 +128,17 @@
   :hook
   (eshell-before-prompt-hook . (lambda ()
                                  (setq xterm-color-preserve-properties t)))
-  :config
-  (setenv "TERM" "xterm-256color"))
+  ;; Scope `TERM=xterm-256color' to eshell-spawned processes only by
+  ;; binding the env var on the eshell mode hook.  The previous global
+  ;; `setenv' at config-time changed `process-environment' for the
+  ;; whole Emacs process, so every subsequent `start-process' inherited
+  ;; `xterm-256color' regardless of whether the receiver was a terminal
+  ;; that could actually interpret the escapes.
+  :hook
+  (eshell-mode . (lambda ()
+                   (setq-local process-environment
+                               (cons "TERM=xterm-256color"
+                                     process-environment)))))
 
 (use-package eshell-syntax-highlighting
   :after esh-mode