add HarborForge.PlexumPlugin submodule

Plexum-side counterpart to HarborForge.OpenclawPlugin. Pins to the
plugin repo's main branch (initial commit). Tracked separately from
HarborForge.OpenclawPlugin — the two plugins run side by side during
the OpenClaw → Plexum migration window.

Operator install:

  cd HarborForge.PlexumPlugin
  bash scripts/install.sh

writes a binary + manifest into ~/.plexum/plugins/harbor-forge/.

.gitmodules

@@ -24,3 +24,6 @@
 	path = HarborForge.Cli
 	url = https://git.hangman-lab.top/zhi/HarborForge.Cli.git
 	branch = main
+[submodule "HarborForge.PlexumPlugin"]
+	path = HarborForge.PlexumPlugin
+	url = https://git.hangman-lab.top/zhi/HarborForge.PlexumPlugin.git