feat: paste local files into chat attachments

Copied local files previously required the attachment picker. The chat
composer now imports one or more supported files directly from paste,
sharing parsing progress, limits, and batch rollback with file selection.
Text and screenshot paste remain available, and failures preserve drafts.

Validation: prior 166 focused tests, typecheck, and lint passed; 28 targeted
tests passed again before commit. The prior full suite had 4214 passes,
67 skips, and 6 window-recovery failures; window tests passed 10/10 alone.
Native clipboard behavior was verified on Windows; macOS/Linux remain unverified.

Release note: 现在可在文件管理器中复制一个或多个支持的本地文件，直接粘贴到聊天输入框添加附件；可查看读取和解析进度，失败时保留草稿与已有附件。

FEATURES.md

@@ -47,7 +47,10 @@ otherwise.
   available. Conversation search has an inline clear action that restores the
   scoped list and returns focus to the input.
 - [x] **File, screenshot, window, and clipboard context**: Added to model
-  context only after explicit user selection.
+  context only after explicit user selection. Paste one or more supported local
+  files into the chat composer with `Ctrl+V` to add attachments, using the same
+  parsing progress and limits as the attachment button. Text and screenshot
+  paste remain available. See [attachment rules](./docs/features/document-processing/prd.md#321-聊天附件入口).
 - [x] **Per-file workspace diffs**: Changed files expose separate staged and
   unstaged diffs, including deleted, renamed, and untracked files. Refresh
   reloads the selected diff, browsed directory, and expanded directories; more than 50 changes