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/usage.md

@@ -24,8 +24,28 @@ imagen version                      print version
 | `--negative`   | empty                        | Negative prompt (ignored by some adapters)                  |
 | `--output`     | empty (= use naming template) | Explicit path                                              |
 | `--no-sidecar` | `false`                      | Skip the JSON sidecar even if config enables it             |
+| `--preview`    | (auto)                       | Force open a tmux preview window via `tmux-img`             |
+| `--no-preview` | (auto)                       | Suppress the preview window (use for batch / CI callers)    |
 | `--config`     | `~/.config/imagen.yaml`      | Override config path                                        |
 
+### Preview window
+
+After a successful generate, imagen optionally opens a sibling tmux window
+named `img:<slug>` running `tmux-img --hold <path>`. The new window is
+spawned in the background (`tmux new-window -d`) so the generating pane
+keeps focus and its terminal output.
+
+Resolution order is **config → `$IMAGEN_PREVIEW` → flag** (later wins):
+
+- `output.preview` in `imagen.yaml`: `auto` (default) | `on` | `off`
+- `IMAGEN_PREVIEW=auto|on|off` overrides config
+- `--preview` / `--no-preview` override env
+
+`auto` previews iff stdout is a TTY *and* `$TMUX` is set. `on` previews
+unconditionally and errors outside a tmux session. `off` never previews.
+
+Preview failures are non-fatal — the image already wrote.
+
 ## Examples
 
 ```sh