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/README.md

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+# Platform packages
+
+Templates for the `@impeccable/cli-<os>-<arch>` packages the `impeccable` npm
+shim (`cli/bin/cli.js`) declares as `optionalDependencies`. npm installs only
+the one matching the host (`os` / `cpu` fields), and the shim resolves
+`<package>/bin/impeccable[.exe]` from it before falling back to the user cache
+or a download.
+
+They are **published by the engine release**, not from this repo: the release
+pipeline copies each template, sets `version` to the engine version, drops the
+built binary at `bin/impeccable[.exe]` (executable), and publishes it under
+`@impeccable`. The version pinned in this repo's `package.json`
+`optionalDependencies` must equal `ENGINE_VERSION`.
+
+Targets: `darwin-arm64`, `darwin-x64`, `linux-x64`, `linux-arm64`, `windows-x64`.

cli/platform-packages/darwin-arm64/package.json

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+{
+  "name": "@impeccable/cli-darwin-arm64",
+  "version": "0.0.0-engine",
+  "description": "impeccable engine binary for darwin-arm64. 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": ["darwin"],
+  "cpu": ["arm64"],
+  "files": ["bin/", "LICENSE"],
+  "bin": {
+    "impeccable-darwin-arm64": "bin/impeccable"
+  }
+}

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

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+{
+  "name": "@impeccable/cli-darwin-x64",
+  "version": "0.0.0-engine",
+  "description": "impeccable engine binary for darwin-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": ["darwin"],
+  "cpu": ["x64"],
+  "files": ["bin/", "LICENSE"],
+  "bin": {
+    "impeccable-darwin-x64": "bin/impeccable"
+  }
+}

cli/platform-packages/linux-arm64/package.json

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+{
+  "name": "@impeccable/cli-linux-arm64",
+  "version": "0.0.0-engine",
+  "description": "impeccable engine binary for linux-arm64. 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": ["arm64"],
+  "files": ["bin/", "LICENSE"],
+  "bin": {
+    "impeccable-linux-arm64": "bin/impeccable"
+  }
+}

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

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+{
+  "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"
+  }
+}

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

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+{
+  "name": "@impeccable/cli-windows-x64",
+  "version": "0.0.0-engine",
+  "description": "impeccable engine binary for windows-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": ["win32"],
+  "cpu": ["x64"],
+  "files": ["bin/", "LICENSE"],
+  "bin": {
+    "impeccable-windows-x64": "bin/impeccable.exe"
+  }
+}