Support Dropbox references in get

[AndreyVMarkelov]

Summary

• allow get sources to use Dropbox paths, file IDs (id:), revisions (rev:), and namespace-relative paths (ns:)
• preserve opaque Dropbox references unchanged while continuing to normalize ordinary paths
• derive default local names and recursive layout from Dropbox metadata
• centralize common metadata field access and update generated command documentation

Why

get previously normalized every source as a path, which prepended / to revision and identifier references. Passing supported Dropbox reference forms through unchanged enables downloading historical revisions and items addressed by ID or namespace.

User impact

Users can now run commands such as:

dbxcli get rev:a1c10ce0dd78 ./historical-file.txt

When the local target is omitted or is an existing directory, get uses the filename returned by Dropbox metadata.

Validation

• gofmt
• go vet ./...
• go test ./... -count=1
• go test -race ./...
• go build ./...
• golangci-lint v2.12.2 (0 issues)
• govulncheck ./... (no vulnerabilities)
• actionlint
• generated command documentation and JSON schemas
• live read-only manual tests for path, relative path, rev:, id:, and ns: sources; explicit/default/directory/stdout targets; text/JSON output; and recursive path/ID/namespace downloads
• byte-for-byte checksum comparison across path, revision, ID, namespace, and stdout downloads