[AndreyVMarkelov]

Adds put --if-exists skip-same: upload missing files, skip identical files, and return a path conflict when contents differ or the remote content hash is unavailable. Rerunning dbxcli put report.txt /report.txt --if-exists skip-same skips an unchanged file without transferring its contents.

Uses the existing SDK contenthash.Compute and checks size before hashing. Supports single files, recursive uploads, and stdin; missing destinations use strict add mode to prevent overwriting concurrently created files. Updates JSON help/manifest enums, the public JSON schema, and generated command documentation. Dry-run retains its existing planning behavior without comparing contents. Comparison uses the fetched metadata snapshot and is not atomic with concurrent file changes.

Validation:

• go test ./...
• go vet ./...
• go build ./...
• Regenerated docs and JSON schemas; confirmed a second generation produces no changes.
• Live Dropbox E2E: missing/identical files, unchanged revision after skip, differing hashes/sizes with remote contents preserved, empty files, stdin, recursive mixed uploads, dry-run, and a 33 MiB session upload followed by a skip. All temporary remote test directories were removed.

Fixes #129.