Project panel: Replace with Precomposition (used N times)

Right-clicking a footage item or solid in the Project panel offers
"Replace with Precomposition (used N times)": the item goes into a new
composition of its size, pixel aspect and frame rate, in the same folder, and
every layer in every composition that used it switches to that composition,
keeping timing, transforms, effects and masks. One undo step. Agents run it as
`file.replaceWithPrecomp`.

The Project panel's details header also shows how often the selected item is
used ("Name ▾, used 3 times"); the ▾ lists and opens the comps that use it.
`Project::item_users` and the `project.usedIn` query back both.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/agents.md

@@ -227,6 +227,9 @@ build real projects through these interfaces; they are worked examples of everyt
 * **Several footage items at once**: `file.newCompFromSelection {single, dimensionsFrom, duration,
   sequence, overlap, overlapDuration, transition, addToRenderQueue}` is one undo step (select the
   items first: `project.select`).
+* **Where an item is used**: `project.usedIn {item?}` → `{item, count, comps: [{id, name, layers}]}`
+  counts the layers, in every comp, whose source the item is (direct uses only, as the Project
+  panel's "Name ▾, used 3 times" header; its ▾ lists the comps and opens the one picked).
 * **Locked layers** are left alone: `edit.clear`, `layer.arrange`, `layer.transform` and
   `layer.centerAnchor` skip them and fail when every target is locked ("… is locked"); Select
   All and `layer.selectNext` / `selectPrevious` step over them. Unlock with