Add deprecation warnings + migration guide

[osvaldoandrade]

Guide consumers through migration.

Goal: Clear, actionable migration instructions.

Implementation:

• Add deprecation comments to all aliased types
• Create docs/MIGRATION.md:
  • Old import → new import table
  • Example code changes
  • Timeline: aliases removed in version X+1
• Announce in CHANGELOG

Acceptance criteria:

• Deprecation visible in godoc
• Migration guide complete
• Consumers have 1 release to migrate

Part of: #656
Requires: Strangler-fig implemented