mAi: #5 - tmux-window preview for generate

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.

docs/architecture.md

@@ -15,6 +15,7 @@ upstream API. Each adapter only ever sees its own slice of `imagen.yaml`.
         │   internal/prompt     │   style preset → prompt suffix
         │   internal/output     │   filename templating, sidecar
         │   internal/config     │   YAML loader, validation
+        │   internal/preview    │   tmux-img window spawner
         └──────────┬────────────┘
                    │
         ┌──────────▼────────────┐