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>

.gitignore

@@ -1,6 +1,15 @@
-build
-**.zip
-out.ppm
+# Generated build tree: LLVM IR, objects, compiled apps and the docs site all
+# land under build/ (see `bin/x build` / `bin/x clean`). Root-anchored so a
+# source dir named "build" elsewhere is never accidentally ignored.
+/build/
+
+# Headless render dropped in the working directory by a --headless run.
+/out.ppm
+
+# Archives are never tracked: the Kenney art is committed *extracted* under
+# assets/kenney/**/ (PNG/tmx/License), while the original download .zips and any
+# packaged plugin .zip are local-only build inputs/outputs.
+*.zip
 
 # the toolchain binaries (ludicc, ludic, x, ludic-fmt, ludic-lsp) — all built
 # into bin/ by the one-line bootstrap + `bin/x build`; never checked in. The
@@ -15,7 +24,7 @@ tools/editors/jetbrains/build/
 
 # IntelliJ plugin SDK sandbox (tools/editors/jetbrains)
 .intellijPlatform/
-.idea
+.idea/
 
 # macOS Finder metadata
 .DS_Store