fix(maven): remove unpublished fat-agent module

[jbachorik]

Closes #879

Summary

• remove the unpublished in-repository Maven fat-agent module and goal from the Gradle project graph
• remove its obsolete descriptor, implementation, tests, dependencies, and demo POM
• remove Maven fat-agent claims from public documentation and direct users to the supported Gradle plugin

Why removal

The module was never part of a public BTrace release and depends on three incompatible pre-3.0 contracts: split agent and boot artifacts, an impl-classified extension artifact that is not published, and a classdata location the runtime does not load. Retaining even a failing compatibility stub would publish and advertise a surface that does not need compatibility.

Verification

• full non-integration build on current develop: 322 tasks, passed
• repository-wide spotlessCheck: passed
• Gradle projects confirms btrace-maven-plugin is absent
• no live source or public-document reference to the removed module remains
• git diff --check: passed

---

This change is