Import: only dropped files default to stills; scripts and agents naming a frame still get its run (#431)

The UI behaves as in After Effects (the Import dialog's Sequence checkbox
starts off, dropped files are stills, dropped folders make sequences), but
file.import without sequence or drag keeps importing a numbered still's
run, so existing scripts and agents keep working.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QiAJeFttZp83Fh7YuzAR5f

docs/agents.md

@@ -251,10 +251,10 @@ build real projects through these interfaces; they are worked examples of everyt
   then `keys.velocity {"outSpeed":0,"outInfluence":33.33}`.
 * **Gradients**: `set_property {"path":"contents/gfill/colors","value":["#0080ff","#ffff00"]}` or
   `{"colors":[[0,"#0080ff"],[1,[1,1,0]]],"opacities":[[0,1],[1,0.5]]}`.
-* **Image sequences**: `file.import {"paths":["/r/shot_0001.png"], "sequence": true}` imports the
-  file's whole numbered run as one item (several picked frames: that range; a folder, even without
-  `sequence`: each run in it; without `sequence` picked files import as stills, as in After
-  Effects; `"alphabetical": true`: Force Alphabetical Order, the files
+* **Image sequences**: `file.import {"paths":["/r/shot_0001.png"]}` imports the file's whole
+  numbered run as one item (several picked frames: that range; a folder: each run in it;
+  `"sequence": false`: one still; dropped files, `"drag": true`, import as stills unless
+  `"sequence": true`, as in After Effects; `"alphabetical": true`: Force Alphabetical Order, the files
   play one after another) at `frameRate` or 30 fps (Settings ▸ Import). Missing frames show
   colour bars, as in After Effects. `file.interpretFootage {"items":[id], "frameRate":12,
   "startTimecode":1001}` conforms them and sets Override Start (the item and layers that ran to its end get the new