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/main.ludic

@@ -27,6 +27,7 @@ program X {
     print("  x app <file.ludic> [--headless] [--save-temps]")
     print("                                compile a Ludic program to a native app in build/")
     print("  x tools [--install] [--test]  build the editor toolchain (ludic-fmt, ludic-lsp)")
+    print("  x clean                       remove build/, out.ppm and stray artifacts (keeps bin/)")
     print("")
     print("test:")
     print("  x test                        the full regression suite")
@@ -56,6 +57,7 @@ program X {
     if (cmd == "build-cli") { exit(cmd_build_cli()) }
     if (cmd == "app") { exit(cmd_app()) }
     if (cmd == "tools") { exit(cmd_tools()) }
+    if (cmd == "clean") { exit(cmd_clean()) }
     if (cmd == "test") { exit(cmd_test()) }
     if (cmd == "selfhost-test") { exit(cmd_selfhost_test()) }
     if (cmd == "test-tools") { exit(cmd_test_tools()) }