docs: rename to lowercase kebab-case; move STYLEGUIDE under docs/styleguides

Pure git mv. No content changes — the next commit updates cross-refs.

- STYLEGUIDE.md           → docs/styleguides/bash_styleguide.md
- docs/BUILDING.md        → docs/building.md
- docs/CONFIGURATION.md   → docs/configuration.md
- docs/DECISIONS.md       → docs/decisions.md
- docs/TROUBLESHOOTING.md → docs/troubleshooting.md

Brings docs/ filenames in line with the conventions in
docs/styleguides/docs_styleguide.md: lowercase kebab-case throughout,
styleguides under docs/styleguides/.

Co-Authored-By: Claude <claude@anthropic.com>

docs/troubleshooting.md

@@ -80,7 +80,7 @@ If the global hotkey (Ctrl+Alt+Space) doesn't work, ensure you're not running in
 
 **Note:** Native Wayland mode doesn't support global hotkeys due to Electron/Chromium limitations with XDG GlobalShortcuts Portal.
 
-See [CONFIGURATION.md](CONFIGURATION.md) for more details on the `CLAUDE_USE_WAYLAND` environment variable.
+See [configuration.md](configuration.md) for more details on the `CLAUDE_USE_WAYLAND` environment variable.
 
 ### Keyboard Input Doesn't Work (IBus / GTK Input Method)
 
@@ -237,7 +237,7 @@ If Cowork fails with a VM timeout, the KVM backend is selected but the guest VM
 COWORK_VM_BACKEND=bwrap claude-desktop
 ```
 
-See [CONFIGURATION.md](CONFIGURATION.md#cowork-backend) for how to make this permanent.
+See [configuration.md](configuration.md#cowork-backend) for how to make this permanent.
 
 ### Cowork: virtiofsd not found (Fedora/RHEL)