docs(#860): link Building From Source wiki page from README

Wiki page is now live at github.com/Deep-CodeAI/Agents.KT/wiki/Building-From-Source.
Added a one-line pointer right under the existing 'Getting Started'
quick-build snippet so contributors who want the deeper guide
(integration tests, pitest, updateVerificationMetadata, IDE setup,
troubleshooting) can find it without scrolling.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Skobeltsyn

README.md

@@ -1169,6 +1169,8 @@ cd Agents.KT
 ./gradlew test
 ```
 
+For the full contributor guide — running the live-LLM and MCP integration tests, mutation testing, the dependency-verification workflow, and IDE setup — see the [**Building From Source**](https://github.com/Deep-CodeAI/Agents.KT/wiki/Building-From-Source) wiki page.
+
 ---
 
 ## Roadmap