feat: support reading message text from stdin pipe

[lewiszlw]

When neither --text nor --file is provided, the send command now reads text from stdin if it is a pipe or redirected file.

Behavior:

• --text takes priority over stdin when explicitly provided
• --file takes priority over stdin when explicitly provided
• If neither --text nor --file is given and stdin is a tty, a clear error is returned instead of waiting for interactive input
• Empty piped stdin produces a clear error message

Changes:

• Extract resolve_send_content() with injectable stdin reader for testability
• Add SendContent enum to represent text vs file content
• Update CLI ArgGroup to make text/file optional (still mutually exclusive)
• Expand send help text with Message Sources section
• Add 6 unit tests covering all stdin behavior combinations
• Update README.md and SKILL.md with stdin examples and priority rules