docs: Consolidate migration guide into single AI-focused document

- Merge migration spec into how-to/migrate-from-0x.md
- Remove reference/specs/migration-2.0.md
- Keep comprehensive content: Why Migrate, all phases, AI prompts
- Streamlined Quick Start with CLAUDE.md template
- Each phase has dedicated AI prompt for agent execution
- Update mkdocs.yaml, llms.txt, specs/index.md

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

mkdocs.yaml

@@ -97,8 +97,6 @@ nav:
               - Data Manipulation: reference/specs/data-manipulation.md
               - AutoPopulate: reference/specs/autopopulate.md
               - Job Metadata: reference/specs/job-metadata.md
-          - Migration:
-              - Migration 2.0: reference/specs/migration-2.0.md
       - Configuration: reference/configuration.md
       - Definition Syntax: reference/definition-syntax.md
       - Errors: reference/errors.md