CLI: turn the impeccable npm package into a platform-binary shim

cli/engine, cli/lib, and cli/bin/commands are gone; their behavior lives in
the engine binary. cli/bin/cli.js now resolves the binary from IMPECCABLE_BIN,
the @impeccable/cli-<os>-<arch> optional dependency (templates under
cli/platform-packages/, published by the engine release), the
~/.impeccable/bin/<version>/ cache, or a checksum-verified download, and
execs it. package.json drops the engine dependencies and the library
exports; puppeteer moves to devDependencies for the icon scripts.
README.npm.md describes the shim.

Prepared with AI assistance (Claude Code).

cli/platform-packages/linux-x64/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "@impeccable/cli-linux-x64",
+  "version": "0.0.0-engine",
+  "description": "impeccable engine binary for linux-x64. Installed as an optional dependency of the impeccable npm package; do not depend on it directly.",
+  "license": "SEE LICENSE IN LICENSE",
+  "homepage": "https://impeccable.style",
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/pbakaus/impeccable.git"
+  },
+  "os": ["linux"],
+  "cpu": ["x64"],
+  "files": ["bin/", "LICENSE"],
+  "bin": {
+    "impeccable-linux-x64": "bin/impeccable"
+  }
+}