Upload only changed settings files

[sgibler-droid]

Addresses #508.

Summary

This changes the upload flow so normal uploads send only files that are new or changed, plus the cloudSettings metadata file. Forced uploads keep the previous all-files behavior.

The important safety detail is in UpdateGIST: it now separates the files being added/updated from the complete local file set used for deletion decisions. Without that separation, passing only changed files could accidentally mark unchanged remote gist files for deletion.

Changes

• Added GistChangeService to detect changed/new setting files and derive the upload payload.
• Updated upload flow to send the partial payload during normal uploads.
• Preserved full upload behavior when sync.forceUpload is enabled.
• Updated summary output to list the actual files uploaded in the run.
• Added regression tests for changed-file detection, normal partial upload, and forced full upload.

Verification

Ran locally:

npm run compile
npm run tslint-check
npx tsc -p ./
./node_modules/.bin/mocha --recursive "./out/test/service/gistChange/**/*.js"

Results:

compile: pass
lint: pass
typecheck: pass
targeted tests: 3 passing

I also verified the existing full test suite locally with a VS Code runtime shim, since this older test harness imports vscode under Mocha.