Custom prompt args (numeric) (#4470)

[Cherry picked from /pull/3565]

Adds $1, $2, $3, $ARGUMENTS param parsing for custom prompts.

docs/prompts.md

@@ -6,6 +6,11 @@ Save frequently used prompts as Markdown files and reuse them quickly from the s
 - File type: Only Markdown files with the `.md` extension are recognized.
 - Name: The filename without the `.md` extension becomes the slash entry. For a file named `my-prompt.md`, type `/my-prompt`.
 - Content: The file contents are sent as your message when you select the item in the slash popup and press Enter.
+- Arguments: Local prompts support placeholders in their content:
+  - `$1..$9` expand to the first nine positional arguments typed after the slash name
+  - `$ARGUMENTS` expands to all arguments joined by a single space
+  - `$$` is preserved literally
+  - Quoted args: Wrap a single argument in double quotes to include spaces, e.g. `/review "docs/My File.md"`.
 - How to use:
   - Start a new session (Codex loads custom prompts on session start).
   - In the composer, type `/` to open the slash popup and begin typing your prompt name.