perf(deps): move pi-coding-agent to optional peerDependencies

Reduces node_modules from 312MB to 154MB. The import is type-only
(import type { ExtensionAPI }) so the 163MB package was installed
but never executed at runtime. End users get the lean install;
contributors still get it via devDependencies during development.

package.json

@@ -47,7 +47,6 @@
     ]
   },
   "devDependencies": {
-    "@earendil-works/pi-coding-agent": "0.75.5",
     "@types/node": "25.6.0",
     "prettier": "^3.5.0",
     "tsx": "4.21.0",
@@ -55,5 +54,13 @@
   },
   "dependencies": {
     "@earendil-works/pi-ai": "0.75.5"
+  },
+  "peerDependencies": {
+    "@earendil-works/pi-coding-agent": "^0.75.5"
+  },
+  "peerDependenciesMeta": {
+    "@earendil-works/pi-coding-agent": {
+      "optional": true
+    }
   }
 }