refactor: split env.go handlers into domain files

[xaaha]

Summary

• Split pkg/userFlags/env.go (918 lines) into 6 focused handler files, each with co-located command factory and handler functions
• Created env_crud.go, env_list.go, env_edit.go, env_keys.go, env_recipients.go, env_sync.go
• Split env_test.go (924 lines) into matching test files: env_crud_test.go, env_list_test.go, env_edit_test.go
• Added newEnv*Cmd() factories to env_rotate_key.go and env_migrate.go
• Shrunk subcommands.go from 784 to 450 lines — newEnvCmd() is now a clean assembly list
• Moved StdoutHeaders to pkg/utils/table.go (exported, reusable next to PrintTable)
• Deleted env.go entirely — shared constants moved to their respective consumer files

Zero behavior changes. 88/88 tests pass. mise check all green.

Part of epic #125.

Test plan

• mise check passes (lint, vet, tests)
• 88/88 existing tests still pass — no test logic changed
• hulak env set/get/delete/list/keys/edit all work as before
• hulak env import-key/export-key work as before
• hulak env add-recipient/remove-recipient/list-recipients work as before
• hulak env rotate/rotate-key/migrate work as before