Add resumable migration state

[aureamunoz]

Problem

If a migration session is interrupted (context window full, session timeout, crash), all progress is lost. The user has to start over or manually figure out where things left off.

Proposal

Persist migration state to a file (migration-context.json) in the project root after each module completes. On startup, the skill checks if this file exists and offers to resume from the last completed module.

State to persist

• Strategy chosen (spring-compat / full-quarkus)
• Modules completed (with timestamps)
• Modules skipped (with reason)
• Current module in progress
• Gate results for each module
• Compile status after each module

Behavior

• If migration-context.json exists at startup → ask user: "Resume from module X?" or "Start fresh?"
• After each module completes → update the file
• After full migration completes → keep the file as a record (don't delete)

Reference

• PR feat: add spring to quarkus skill with validators #37 implements this as migration-metadata/migration-context.json