refactor: slim session-start hook from 55 to ~20 lines of injected context

Create ACTIVATION.md as minimal stub for session-start injection.
Full SKILL.md stays in place for on-demand loading when commands are invoked.

hooks/session-start

@@ -7,7 +7,7 @@ const path = require("path");
 
 try {
   const pluginRoot = path.resolve(__dirname, "..");
-  const skillFile = path.join(pluginRoot, "skills", "using-archeflow", "SKILL.md");
+  const skillFile = path.join(pluginRoot, "skills", "using-archeflow", "ACTIVATION.md");
 
   if (!fs.existsSync(skillFile)) {
     console.log("{}");