Sort documentation by genre and document the layout

Task procedure lives in agents/skills/ (migrations.md moves there),
system-shape reference in docs/ (AUDIO-TUNING.md renamed to match),
end-user documentation in manual/. AGENTS.md now states the split.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

default/systemd/user/omarchy-speaker-tuning.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Omarchy speaker tuning filter-chain
-Documentation=https://github.com/basecamp/omarchy/blob/master/docs/AUDIO-TUNING.md
+Documentation=https://github.com/basecamp/omarchy/blob/master/docs/audio-tuning.md
 # WirePlumber does the linking, so starting before it is up risks the output being
 # linked before the speaker device has been discovered.
 After=pipewire.service wireplumber.service