Replace shell-based install with in-process binary downloader

Fetches the platform-specific native binary directly — npm registry
first (sha512-verified tarball, streamed through a minimal in-tree
tar parser) with the Anthropic CDN as a fallback (sha256-verified raw
binary). Writes to context.globalStorageUri/bin/claude[.exe] and
auto-sets executable.path so there's no PATH, sudo, PowerShell
execution-policy, npm EACCES, or Node-version failure modes left.

Adds typed DownloaderError codes (UNSUPPORTED_PLATFORM / NETWORK /
INTEGRITY / WRITE / CANCELLED / AGGREGATE) and threads source /
version / npmCode / cdnCode into analytics so install failures are
finally bucketable instead of collapsed under "the shell command
failed". Error messages are scrubbed of user paths to keep analytics
PII-free.

Fixes a Windows path-with-spaces edge case by skipping cmd.exe shell
wrapping when spawning claude with an absolute executable.path.

Ships 28 tests: 24 unit (tar parser, platform detection, error
helpers) and 4 integration that hit real npm + real CDN end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.vscodeignore

@@ -16,3 +16,4 @@ node_modules
 mcp-permissions.js
 backup-files
 build
+**/test/**