fix: update pi packages to 0.51.0, remove bogus type augmentation

- Update @mariozechner/pi-agent-core, pi-ai, pi-coding-agent, pi-tui to 0.51.0
- Delete src/types/pi-coding-agent.d.ts (declared additionalExtensionPaths which SDK never supported)
- Fix ToolDefinition.execute signature (parameter order changed in 0.51.0)
- Remove dead additionalExtensionPaths from createAgentSession calls

package.json

@@ -159,10 +159,10 @@
     "@homebridge/ciao": "^1.3.4",
     "@line/bot-sdk": "^10.6.0",
     "@lydell/node-pty": "1.2.0-beta.3",
-    "@mariozechner/pi-agent-core": "0.50.9",
-    "@mariozechner/pi-ai": "0.50.9",
-    "@mariozechner/pi-coding-agent": "0.50.7",
-    "@mariozechner/pi-tui": "0.50.7",
+    "@mariozechner/pi-agent-core": "0.51.0",
+    "@mariozechner/pi-ai": "0.51.0",
+    "@mariozechner/pi-coding-agent": "0.51.0",
+    "@mariozechner/pi-tui": "0.51.0",
     "@mozilla/readability": "^0.6.0",
     "@sinclair/typebox": "0.34.48",
     "@slack/bolt": "^4.6.0",