fix: autoformat pasted markdown in inline editor (#2673)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - The inline markdown editor (MarkdownEditor / MDXEditor) is used to
edit agent instructions, issue descriptions, and other content
> - When users paste agent instructions copied from terminals or
consoles, extra leading whitespace is uniformly added to every line
> - PR #2572 fixed markdown structure preservation on paste but did not
address the leading whitespace (dedent) problem
> - This pull request adds a Lexical paste normalization plugin that
strips common leading whitespace and normalizes line endings before
MDXEditor processes pasted content
> - The benefit is that pasted content from terminals/consoles renders
correctly without manual cleanup

## What Changed

- **`ui/src/lib/normalize-markdown.ts`** — Pure utility that computes
minimum common indentation across non-empty lines and strips it
(dedent), plus CRLF → LF normalization
- **`ui/src/lib/paste-normalization.ts`** — Lexical `PASTE_COMMAND`
plugin at `CRITICAL` priority that intercepts plain-text pastes,
normalizes the markdown, and re-dispatches cleaned content for MDXEditor
to process. Skips HTML-rich pastes.
- **`ui/src/components/MarkdownEditor.tsx`** — Registers the new plugin;
updates PR #2572's `handlePasteCapture` to use `normalizeMarkdown()`
(dedent + CRLF) instead of `normalizePastedMarkdown()` (CRLF only) for
the markdown-routing path
- **`ui/src/lib/paste-normalization.test.ts`** — 9 unit tests covering
dedent, CRLF normalization, mixed indent, empty lines, single-line
passthrough, and edge cases

## Verification

- `pnpm --dir ui exec vitest run src/lib/paste-normalization.test.ts` —
9 tests pass
- Manual: paste indented agent instructions from a terminal into any
inline markdown editor and confirm leading whitespace is stripped

## Risks

- Low risk. The plugin only activates for plain-text pastes (no HTML
clipboard data). HTML/rich pastes pass through unchanged. Single-line
pastes are not modified. The dedent logic is conservative — it only
strips whitespace common to all non-empty lines.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [ ] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/components/MarkdownEditor.test.tsx

@@ -81,6 +81,10 @@ vi.mock("../lib/mention-deletion", () => ({
   mentionDeletionPlugin: () => ({}),
 }));
 
+vi.mock("../lib/paste-normalization", () => ({
+  pasteNormalizationPlugin: () => ({}),
+}));
+
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
 (globalThis as any).IS_REACT_ACT_ENVIRONMENT = true;
 

ui/src/components/MarkdownEditor.tsx

@@ -33,7 +33,9 @@ import { AgentIcon } from "./AgentIconPicker";
 import { applyMentionChipDecoration, clearMentionChipDecoration, parseMentionChipHref } from "../lib/mention-chips";
 import { MentionAwareLinkNode, mentionAwareLinkNodeReplacement } from "../lib/mention-aware-link-node";
 import { mentionDeletionPlugin } from "../lib/mention-deletion";
-import { looksLikeMarkdownPaste, normalizePastedMarkdown } from "../lib/markdownPaste";
+import { looksLikeMarkdownPaste } from "../lib/markdownPaste";
+import { normalizeMarkdown } from "../lib/normalize-markdown";
+import { pasteNormalizationPlugin } from "../lib/paste-normalization";
 import { cn } from "../lib/utils";
 
 /* ---- Mention types ---- */
@@ -326,6 +328,7 @@ export const MarkdownEditor = forwardRef<MarkdownEditorRef, MarkdownEditorProps>
       linkPlugin({ validateUrl: isSafeMarkdownLinkUrl }),
       linkDialogPlugin(),
       mentionDeletionPlugin(),
+      pasteNormalizationPlugin(),
       thematicBreakPlugin(),
       codeBlockPlugin({
         defaultCodeBlockLanguage: "txt",
@@ -520,7 +523,7 @@ export const MarkdownEditor = forwardRef<MarkdownEditorRef, MarkdownEditorProps>
     if (!looksLikeMarkdownPaste(rawText)) return;
 
     event.preventDefault();
-    ref.current.insertMarkdown(normalizePastedMarkdown(rawText));
+    ref.current.insertMarkdown(normalizeMarkdown(rawText));
   }, []);
 
   return (