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

src/agents/pi-embedded-runner/compact.ts

@@ -374,7 +374,8 @@ export async function compactEmbeddedPiSessionDirect(
         settingsManager,
         minReserveTokens: resolveCompactionReserveTokensFloor(params.config),
       });
-      const additionalExtensionPaths = buildEmbeddedExtensionPaths({
+      // Call for side effects (sets compaction/pruning runtime state)
+      buildEmbeddedExtensionPaths({
         cfg: params.config,
         sessionManager,
         provider,
@@ -396,7 +397,6 @@ export async function compactEmbeddedPiSessionDirect(
         thinkingLevel: mapThinkingLevel(params.thinkLevel),
         tools: builtInTools,
         customTools,
-        additionalExtensionPaths,
         sessionManager,
         settingsManager,
       });