feat(core): add tmux plugin manager entry to chezmoiexternal configuration

.chezmoiexternal.toml

@@ -15,3 +15,11 @@
     url = "https://github.com/tellnobody1/sublime-purescript-syntax.git"
     [".config/bat/syntaxes/sublime-purescript-syntax".pull]
         args = ["--ff-only"]
+
+# tmux plugin manager — tmux.conf runs it from this path. tpm installs the
+# other plugins next to it (prefix + I); those stay unmanaged.
+[".config/tmux/plugins/tpm"]
+    type = "git-repo"
+    url = "https://github.com/tmux-plugins/tpm.git"
+    [".config/tmux/plugins/tpm".pull]
+        args = ["--ff-only"]