OK, it's a sad day for me

Current state:
1) Doesn't response to power button
2) Can't get into flash mode (I guess I've tried every combination of connecting the cable, inserting charged battery, holding 'u' button)
3) Tried to cold-flash, with no success, there isn't any sign of connection in dmesg
4) When connecting phone with pc or wall charger without battery, there is constant green light; when I insert battery while connected, the led turns off; when I connect while battery is inside, the light is solid orange
5) no sign of activity on the screen

What I was doing:
Few days ago i flashed it with latest FIASCO and VANILLA to clean it. Then I installed kernel power from repositories, v42. CPU was clocked to 500MHz. I started installing Debian image. The only opened applications were terminal where image was being extracted, conky and batterygraph to see what's going on. Phone was plugged to wall charger.
Suddenly it rebooted. Fully, but with information that 'custom kernel could not be loaded' or something similar. So I rebooted phone manually to see what will happen. It didn't get pass moving dots and then it died completely.

I thought it was something with battery. Maybe while extracting the image it was so overloaded that it couldn't even load the battery. It's silly but that's what i thought to myself. And I was right partially because the battery was drained out but the situation didn't change when I had installed a fully charged one. Battery and charger are original Nokia's.

Shortly before I enabled testing and devel repos but didn't make it to install anything.

So what you think? Is there any hope? Is there something more I could try?

R.I.P your N900 will be in a better place as spare parts.

u serious?

Assuming it's dead, what happened?
Did I do something wrong?

Use a battery with (almost) full charge. Charge them on an external battery charger or another phone or model that also uses BL-5J. Just google it if you are not sure which one.
The LED signal can be misleading as charging may fail even with the orange light.
Probably your battery was drained or had little charge left after running such heavy tasks. And don't forget that rebooting itself uses a lot of resource and so A LOT OF power.
This symptom has also happened to quite a few devices that cannot taake charge.
Just try it. It won't hurt anything. Hope this helps.

PS it's not so easy to REALLY brick your phone.