docs: move service issue notes from READMEs into docs/<service>

Service READMEs now cover only what the service is and how to deploy it.
Known issues, log noise and troubleshooting move to docs/<service>/, named
after the service directory, so editing them never redeploys the service.
Drop alloy's validate workflow, which never ran from a subdirectory.

docs/opencode/known-noise-xdg-open.md

@@ -0,0 +1,11 @@
+# Known noise: `xdg-open` stack trace on start
+
+Applies to `opencode/compose.yml`.
+
+The container logs a Bun stack trace ending in
+`Executable not found in $PATH: "xdg-open"` on every start. `opencode web`
+tries to open the UI in a local browser; there isn't one. It is noise — the
+server is already listening by then, and the container keeps running.
+
+Installing `xdg-utils` would silence it at the cost of pulling X11 in and
+tripling the image, which is not worth it for a log line.