refactor: merge /spark command into /roll

Consolidate the `/spark` command into `/roll` to simplify the CLI.
The `/roll` command now handles both dice notation and spark table
slugs.

- Update `parseInput` to treat `/roll` arguments as a generic string.
- Update `dispatchCommand` to check for spark table matches before
  evaluating dice notation or variable expressions.
- Update command completions to show both dice and spark table
  suggestions under `/roll`.
- Update documentation and UI placeholders to reflect the change.

src/components/RevealManager.tsx

@@ -126,7 +126,7 @@ export const RevealManager: Component = () => {
             rect: sparkTable.getBoundingClientRect(),
             action: () =>
               setActionPrefill({
-                command: "/spark",
+                command: "/roll",
                 text: combinedSlug,
               }),
             title: "Roll spark table",