Fix unnecessary gist rewrites during sync upload

[cedar323]

Summary

• upload only changed gist files instead of resending the full gist payload
• explicitly delete remote-only gist files while preserving keybindings* variants
• show deleted gist files in the upload summary and cover the diff/payload logic with tests

Why

Issue #508 reports that every sync upload rewrites all gist files, which produces unnecessary updates. This change computes a minimal gist patch from the remote gist state and local setting files, then sends only changed entries plus explicit deletions.

Compared with #1461, this also verifies the actual PATCH payload shape and preserves remote keybindings variants from accidental deletion.

Validation

• npx tsc src/service/gistUpdate.ts test/service/gistUpdate.test.ts --module commonjs --target es2018 --lib es2018 --types node,mocha,chai --esModuleInterop --skipLibCheck --outDir .tmp-gistupdate-test
• node .\\node_modules\\mocha\\bin\\mocha .\\.tmp-gistupdate-test\\test\\service\\gistUpdate.test.js