Putting a DH fork on an enduro bike: consequences?

Let us assume that the wheel size stays the same at 29". Let us also assume that a DH fork can be installed on that frame with no clearance or mounting issues. What remains are questions of frame geometry and safety.

How would this specifically affect the bike?

I would say that the main parameter that changes is fork's axle to crown distance. DH forks have more travel than enduro: let's say 200 mm against 170 mm that the bike you link to has. So the top of the bike gets raised up for 30 mm. Each 10 mm of change corresponds roughly to 0,5° decrease of head tube angle. So that makes it 1,5° or more. It means the whole front end would raise up; if you stand on a perfectly horizontal surface, with a new fork it would feel as if you ride 1,5° uphill. Needless to say such a change won't help you climbing up hills, but exactly the opposite.

1,5° is a lot of a change for head tube angle, which becomes 64°. Most likely you will find it harder to make turns or to control your bike in turns.

Bottom bracket would be raised as well, possibly making the bike unstable in controlling, see this answer. Less pedal strikes, however.

What would be other effects?

It would weigh more, and it would not be covered by vendor's guarantee any longer because using a longer travel fork certainly voids it. Remember that an enduro bike frame and remaining components, such as handlebar and wheels, were designed, tested and certified for certain types of drop heights. Downhill riding assumes harsher loads; you would not want your handlebar to snap.

Is this a good idea?

No it is not.

You can get a Rockshox Boxxer 29" DH fork in 180 mm length, that would do the trick.

The main problem that stands out to me is - as strong as Commencal's bikes are - when you slacken the head angle and add more weight to the front end, especially simultaneously, you are putting a ton of stress on your head tube which is likely designed around a specific travel fork. As you could imagine there's already a ton of stress on the head tube as it is and despite their bikes being build like tanks that's a lot more stress so maybe if you ride delicately?