Qualify clipboard image attach as macOS-only in help copy

/help and /paste-image described clipboard attach as universal. The
attach path only reads a macOS PNG; Linux and Windows get an error.
TUI.md now also says the mention popup splices and send inlines.

Author: TheGreatAxios

docs/TUI.md

@@ -604,7 +604,8 @@ so an in-flight lookup cannot reopen it. A lookup that finishes after the
 cursor has left that token does not open. Dismiss clears mention accept state
 and bumps generation.
 Directory picks re-open one level down so the operator can drill into a path
-without retyping it.
+without retyping it. The popup only splices a path into the prompt; submit
+inlines `@file` contents and summarizes `@dir` via `ingestOperatorPrompt`.
 
 A readline-style kill ring backs Ctrl+K/U/W (kill) and Ctrl+Y/Alt+Y
 (yank/yank-pop) on top of the textarea's native delete bindings, which

src/tui/commands/built-in.ts

@@ -102,7 +102,7 @@ export function registerBuiltInCommands(): void {
 
   registerCommand({
     name: "paste-image",
-    description: "Attach the current clipboard image to the next message",
+    description: "Attach a PNG from the macOS clipboard to the next message",
     handler: (_args, _ctx) => ({ type: "paste-image" }),
   });
 

src/tui/keybindings.ts

@@ -70,7 +70,7 @@ export const SHELL_SHORTCUTS: readonly ShellShortcut[] = [
   { keys: "Alt+Y", description: "replace the text just yanked with the next-older kill" },
   {
     keys: "Ctrl+V / Ctrl+P",
-    description: "attach an image from the clipboard to the next message",
+    description: "attach a PNG from the macOS clipboard to the next message",
   },
   {
     keys: "@",