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.

internal/config/config.go

@@ -19,11 +19,16 @@ type Config struct {
 	Backends       map[string]BackendSpec `yaml:"backends"`
 }
 
-// OutputConfig controls where generated images and metadata sidecars land.
+// OutputConfig controls where generated images and metadata sidecars land,
+// and whether `imagen generate` opens a tmux preview window.
 type OutputConfig struct {
 	Directory         string `yaml:"directory"`
 	Naming            string `yaml:"naming"`
 	WriteMetadataJSON bool   `yaml:"write_metadata_json"`
+	// Preview is the tri-state preview mode: "auto" (default), "on", "off".
+	// Empty / unset is treated as "auto". $IMAGEN_PREVIEW and the
+	// --preview/--no-preview flags override this in turn.
+	Preview string `yaml:"preview"`
 }
 
 // BackendSpec is one entry under `backends:`. Type identifies the adapter;
@@ -78,6 +83,11 @@ func (c *Config) Validate() error {
 			return fmt.Errorf("default_backend %q is not defined under backends:", c.DefaultBackend)
 		}
 	}
+	switch c.Output.Preview {
+	case "", "auto", "on", "off":
+	default:
+		return fmt.Errorf("output.preview = %q (must be auto|on|off)", c.Output.Preview)
+	}
 	for name, spec := range c.Backends {
 		if name == "" {
 			return errors.New("empty backend name")
@@ -101,6 +111,11 @@ output:
   directory: ~/Pictures/imagen
   naming: "{date}-{slug}-{seed}.png"
   write_metadata_json: true
+  # Open a tmux window with tmux-img after a successful generation.
+  # auto (default): preview iff stdout is a TTY and $TMUX is set.
+  # on: always preview (errors outside a tmux session).
+  # off: never preview (use this for batch / CI callers).
+  preview: auto
 
 backends:
   flux-schnell-local: