chore(repo): add .editorconfig, reconcile .gitignore, add x clean

Repository hygiene pass (issue #39):

- Add `.editorconfig` mirroring ludic-fmt: 2-space indent, LF, UTF-8, trim
  trailing whitespace and final newline by default; 4-space for Python
  tooling; keep trailing whitespace in Markdown (hard line breaks).
- Reconcile `.gitignore`: root-anchor `/build/` and `/out.ppm`, normalise the
  misleading `**.zip` glob to `*.zip`, and document that the Kenney art is
  tracked *extracted* while the download/plugin zips are local-only. `.idea`
  -> `.idea/`.
- Add a `bin/x clean` command that removes `build/`, the root `out.ppm`, and
  stray `bin/*.tmp`, keeping the toolchain binaries so the running `x`
  survives.

No generated artifacts land outside build/ or bin/, both of which are ignored.

Closes #39

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/x/build.ludic

@@ -81,3 +81,15 @@ function cmd_app() -> int {
   print(`done. run: ./{outbin}   (from the repo root, so assets/ resolves)`)
   return 0
 }
+
+# Remove every generated artifact and leave the tracked source untouched: the
+# whole build/ tree (IR, objects, compiled apps), the headless render dropped in
+# the root (out.ppm), and any stray *.tmp/*.ll left behind by a failed build.
+# The toolchain binaries in bin/ are kept so the running `x` survives; rebuild
+# them any time with `x build`.
+function cmd_clean() -> int {
+  run("rm -rf build")
+  run("rm -f out.ppm bin/*.tmp")
+  print("cleaned: build/, out.ppm, bin/*.tmp  (bin/ toolchain kept — `x build` to rebuild)")
+  return 0
+}