[3.0] Add migration prompt/skill for legacy custom system-support packaging

[jbachorik]

Goal

Before BTrace 3.0.0, provide a reusable prompt/skill that helps users migrate custom system support packaged through legacy libs, configuration files, ad-hoc JARs, and related setup into proper BTrace extensions and masked fat agents.

Scope

The migration assistant should:

• inventory the existing packaging layout, including custom libraries, configs, scripts, service providers, and target-system dependencies
• identify the public probe-facing API versus runtime implementation classes
• recommend the correct extension structure, descriptors, permissions, service-provider declarations, external types, and package shading
• generate or guide the io.btrace.extension Gradle configuration
• generate or guide io.btrace.fat-agent configuration using the masked btraceJar and io.btrace.boot.Loader
• explain bootstrap-visible API classes versus masked/runtime-loaded implementation .classdata
• preserve required configuration/resources and flag unsafe target-application classpath coupling
• provide validation steps for extension artifacts, fat-agent contents, startup, and a first trace
• link to the 3.0 extension-development, interface-rule, fat-agent, and migration documentation

Deliverable

Provide a user-facing migration prompt and, where appropriate, a Codex-compatible skill/workflow. It should begin with discovery questions or repository inspection, produce a concrete migration plan, and optionally scaffold the resulting extension/fat-agent files after user confirmation.

Acceptance criteria

• Handles a representative legacy setup using custom libs and configuration resources
• Produces a correct API/implementation split and extension metadata
• Preserves or creates required META-INF/services provider declarations
• Selects and documents required extension permissions
• Produces a fat agent based on the masked BTrace JAR with Loader entry points
• Includes artifact inspection and live startup/first-trace verification
• Documents common migration failures and classloading pitfalls
• Is referenced from the relevant BTrace 3.0 migration and extension documentation

Release requirement

Target this for completion before the BTrace 3.0.0 release.