Add a 'task trailer t-123' subcommand that emits 'Task-Id: t-123' suitable for use as a git trailer. Also ensure 'task show --json' output includes the trailer string so agents can pipe it into commit messages. Convention: every commit associated with a task must have the Task-Id trailer.
Implemented git trailer support: added 'task trailer <id>' command that emits 'Task-Id: t-XXX'. Updated task show --json to include trailer/taskTrailer fields for easy piping. Also updated dev workflow docs to use task trailer.