feat(vim): added Vim ex :join and :join! support for line joining

- Implemented parsing of `:j`/`:join` and `:j!`/`:join!` ex commands and mapped them to a new `join` command shape with a whitespace-trim flag.
- Added VimEngine handling for `join` to concatenate addressed lines (or the current plus next line by default), optionally normalizing whitespace and reporting line count in the status message.
- Updated vim prompt guidance, changelog notes, and tests to document and verify both normal and `:join!` join behavior.

packages/coding-agent/src/vim/commands.ts

@@ -294,6 +294,9 @@ export function parseExCommand(input: string, context?: VimExParseContext): VimE
 		const flags = rest.slice(4).trim();
 		return { kind: "sort", range: range ?? undefined, flags };
 	}
+	if (rest === "j" || rest === "join" || rest === "j!" || rest === "join!") {
+		return { kind: "join", range: range ?? undefined, trimWhitespace: !rest.endsWith("!") };
+	}
 
 	if (rest.startsWith("substitute")) {
 		const segments = parseDelimitedSegments(rest.slice("substitute".length));

packages/coding-agent/src/vim/engine.ts

@@ -2298,6 +2298,34 @@ export class VimEngine {
 				this.statusMessage = `Sorted ${endLine - startLine + 1} line${endLine === startLine ? "" : "s"}`;
 				return;
 			}
+			case "join": {
+				const currentLine = this.buffer.cursor.line + 1;
+				const baseRange = this.#resolveExRange(
+					command.range,
+					currentLine,
+					command.range ? undefined : Math.min(this.buffer.lineCount(), currentLine + 1),
+				);
+				const startLine = baseRange.start;
+				const endLine =
+					baseRange.start === baseRange.end ? Math.min(this.buffer.lineCount(), baseRange.end + 1) : baseRange.end;
+				const lineCount = endLine - startLine + 1;
+				if (lineCount < 2) {
+					this.statusMessage = "Nothing to join";
+					return;
+				}
+				await this.#applyAtomicChange([":join"], () => {
+					const startIndex = startLine - 1;
+					if (command.trimWhitespace) {
+						this.buffer.joinLines(startIndex, lineCount - 1);
+						return;
+					}
+					const joined = this.buffer.lines.slice(startIndex, endLine).join("");
+					this.buffer.lines.splice(startIndex, lineCount, joined);
+					this.buffer.setCursor({ line: startIndex, col: Math.max(0, joined.length - 1) });
+				});
+				this.statusMessage = `Joined ${lineCount} lines`;
+				return;
+			}
 			case "append": {
 				const anchorRange = this.#resolveExRange(command.range, this.buffer.cursor.line + 1);
 				const anchorLine = anchorRange.end;

packages/coding-agent/src/vim/types.ts

@@ -140,6 +140,7 @@ export type VimExCommand =
 	| { kind: "copy"; range?: VimLineRange | "all"; destination: number }
 	| { kind: "move"; range?: VimLineRange | "all"; destination: number }
 	| { kind: "sort"; range?: VimLineRange | "all"; flags: string }
+	| { kind: "join"; range?: VimLineRange | "all"; trimWhitespace: boolean }
 	| { kind: "global"; range?: VimLineRange | "all"; pattern: string; command: string; invert: boolean }
 	| { kind: "append"; range?: VimLineRange; text: string }
 	| { kind: "insert-before"; range?: VimLineRange; text: string };