Let the agent use stored files, presets and usages (#431 #432 #433)

The agent got stuck where the editor has it easy: it could not make a
Sample play without a sound asset, it built instruments node by node
although the Preset menu has them ready, and it could not tell what
names an asset before reworking or deleting it.

- list_files is a new tool. It lists the pictures, fonts, sounds or
  videos the server holds, with size, video state and the asset that
  already wears a file. Bakes and stream copies of songs are left out.
- add_file_asset makes an asset of such a file, as the picker does.
  It reuses the create rules of the import pipeline, so the four
  import kinds are exported now. It refuses a file already in the
  project (naming that asset), a picture without a size, a video not
  yet converted and a file the server does not hold.
- run_commands fetches the file lists its steps need before it runs,
  so the commands themselves stay synchronous.
- apply_preset sets one of the 16 instrument presets or 7 sfxr
  presets, like the Preset menu. Presets take every note, so the
  C4 to B4 range of a new Notes node is no trap there.
- get_asset gains usedBy: every asset naming the asset or one of its
  frames or clips, with the object, node, item or key that does, plus
  start scene, master FX and master routing of the project.
- usesOf in src/core/model/usages.ts walks the project data instead of
  a list of fields, so no new field can be missed. The node every
  asset has in the master routing does not count as a use.
- The relay tells the agent about all three ways.

Found on the way: suite 740 for 2e4c0a0 (#430) was red in e2e 68h
only, a sound clip 61 ms early against 50 allowed. #430 did not touch
sound timing, and e2e 68 on the same tree was green on atx (-4 ms).
Taken as a timing wobble under load until the next suite says more.

Tests:
- agent-files (new, 4 tests) with the other agent and relay tests:
  130/130.
- 8 counter checks, all compiling, all red.
- e2e 71 on atx: 26/26, new 71g9 lists an uploaded sound, makes an
  asset of it and fills an instrument from a preset.
- Gate on atx: CHECK EXIT 0, 326 files, 5021 tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/agent/tools.json

@@ -41,10 +41,25 @@
     },
     "annotations": { "readOnlyHint": true, "openWorldHint": false }
   },
+  {
+    "name": "list_files",
+    "title": "List the stored files",
+    "description": "The pictures, typefaces, sounds or videos the server holds for this account, the demo material among them (demo), whether a project uses them or not: each with its file, the name it was uploaded as, its size, a picture's width and height, a video's state, length, frame rate and sound, and the asset already wearing it in this project (asset). The command add_file_asset makes an asset of one; uploading is for the user.",
+    "inputSchema": {
+      "type": "object",
+      "properties": {
+        "type": { "type": "string", "enum": ["texture", "font", "sound", "video"] },
+        "query": { "type": "string", "description": "Only files whose upload name contains this." }
+      },
+      "required": ["type"],
+      "additionalProperties": false
+    },
+    "annotations": { "readOnlyHint": true, "openWorldHint": false }
+  },
   {
     "name": "get_asset",
     "title": "Read an asset",
-    "description": "One asset as stored: its property values, the rows of its properties (key, kind, default), the objects of a scene or container with their names and positions (the content of a placed container too, marked fromContainer), what the rows of an animation may aim at (targets), the cells of a small tilemap, the pixels of small pixel art as rows of palette indices (colors as \"#rrggbb\"), the notes of a pattern with their names and how many ticks it is long, the rows a clip of a sound or video has (clipProperties), how many seconds a loaded sound lasts, and under silent what of a pattern, song or composition is not heard: all of it without an instrument, or the notes its instrument does not sound. A graph is only counted here; read it with get_graph.",
+    "description": "One asset as stored: its property values, the rows of its properties (key, kind, default), the objects of a scene or container with their names and positions (the content of a placed container too, marked fromContainer), what the rows of an animation may aim at (targets), the cells of a small tilemap, the pixels of small pixel art as rows of palette indices (colors as \"#rrggbb\"), the notes of a pattern with their names and how many ticks it is long, the rows a clip of a sound or video has (clipProperties), how many seconds a loaded sound lasts, and under silent what of a pattern, song or composition is not heard: all of it without an instrument, or the notes its instrument does not sound. Under usedBy: every asset naming it or one of its frames or clips, with the objects, nodes, items or keys in there that do (none for a property of the asset itself), and the project's start scene, master FX or Master routing; empty when nothing uses it. A graph is only counted here; read it with get_graph.",
     "inputSchema": {
       "type": "object",
       "properties": { "asset": { "type": "string", "description": "Id of the asset." } },