Add Unix pipe support for put and get

[AndreyVMarkelov]

Summary

• put - <target>: upload from stdin (spooled to temp file, then existing upload path)
• get <source> -: download to stdout (byte-clean, retry before first byte only)
• EPIPE/broken pipe handled gracefully (exit 0, no noisy stack trace)
• Temp file security: 0600 permissions, name independent of target/content
• Cleanup failure after successful upload reports error with temp path on stderr

Closes #194

Test plan

• put - without target errors
• put - /file.txt uploads stdin content
• put - /- uploads to Dropbox path /-
• put ./- /remote uploads local file named -
• put - /folder/ rejects directory-style target
• put - --recursive rejects
• put - /existing-folder rejects when metadata says folder
• Empty stdin creates zero-byte file
• Cleanup failure after upload returns non-zero with sensitive-data warning
• get /file.txt - writes exact bytes to stdout
• get /- - downloads Dropbox path /-
• get /folder - and get -r /folder - reject
• Stdout download creates no local files
• Retry before first byte, no retry after partial output
• Writer EPIPE returns nil (clean exit)
• SIGPIPE ignored during stdout download (Unix)
• Existing put, put -r, get, get -r behavior unchanged