Adds an optional `imagen generate` post-step that opens a sibling
tmux window running tmux-img --hold <path>.
- internal/preview: Mode (auto|on|off), Resolve, and a Spawner that
shells out to tmux new-window. Typed errors for missing tmux,
missing tmux-img, and "preview forced on outside $TMUX".
- cmd/imagen/generate: --preview / --no-preview flags plus
$IMAGEN_PREVIEW. Resolution chain: config -> env -> flag.
auto requires both stdout-is-tty and $TMUX. Failures are
warnings - the image is already on disk.
- internal/config: output.preview field, validated to auto|on|off,
threaded into the sample.
- Tests for ParseMode, Resolve, Spawn argv (incl. shell quoting of
paths with apostrophes), missing-binary errors, and the CLI
resolution table.
- Docs (usage + architecture) updated.
/imagine SKILL.md edit lives in dotfiles - deferred to coordinate
with #4.
