feat: add --no-color flag to explicitly disable colored output
Add a persistent --no-color flag that forces color off for all
subcommands. This complements the automatic environment detection
(NO_COLOR, COG_NO_COLOR, TERM=dumb, non-TTY stderr) added in the
previous commit, giving users an explicit CLI escape hatch.
Refs: https://clig.dev/#output, https://no-color.org/
