Remove cog migrate command and dead code (#2667)
The cog migrate command was a hidden command used to migrate projects
to fast boot compatibility. This removes:
- pkg/cli/migrate.go - CLI command definition
- pkg/migrate/ - entire migration package (7 files)
- pkg/coglog/migrate_log_context.go - migration telemetry context
- python/cog/command/migrate_v1_v1fast.py - Python AST transformer
- Integration tests for migration (3 files)
- Documentation for cog migrate in docs/cli.md and docs/llms.txt
- Migration telemetry methods from pkg/coglog/client.go
