docs: rewrite migration-prompt.ja.md for agent accuracy and user-centric flow

The migration prompt is a meta-prompt that an AI coding agent reads to
migrate a user's v2 codebase to v3. The previous version had several
issues that could lead agents to make mistakes or miss critical steps.

Key changes:
- Remove v3 kit code details per documentation policy (agent has v3 copy)
- Add "files to copy vs files to transform" section so the agent knows
  which files need user-specific conversion
- Use schema.prisma as primary data source for schema conversion instead
  of pg_dump output (structured model definitions are easier to analyze)
- Merge pnpm migration + monorepo restructuring + linter into one phase
  (pnpm-workspace.yaml references apps/*/packages/* so dirs must exist
  before pnpm install; linter catches DSQL-incompatible imports early)
- Add dev DSQL cluster verification step using scripts/dsql.sh before
  touching production database
- Add user-specific analysis steps: custom code inventory, VPC-dependent
  constructs identification, CI/CD pipeline npm/npx cleanup
- Fix phase numbering (1-based, 5 phases) and CDK deploy count (2 required
  deploys in Phase 5, not 4)
- Clarify .npmrc must not be created (strict mode is pnpm default)
- Replace tsc --noEmit with oxlint typeCheck (oxlintrc.json has typeAware)
- Remove redundant "breaking changes reference" section (duplicated phases)
- Consolidate agent behavioral rules at the top with data loss warning

Author: konokenj