fix: preserve start message in /new without workspace name

When user types "/new\nmessage here", the message was being lost
because the parser only extracted startMessage when a workspace name
was provided.

Now correctly extract and pass startMessage, trunkBranch, and runtime
flags even when no workspace name is specified (creation modal flow).

Added tests:
- /new with message but no workspace name
- /new with flags and message but no workspace name

_Generated with `mux`_

Author: ammar-agent

src/utils/slashCommands/parser.test.ts

@@ -179,3 +179,32 @@ describe("commandParser", () => {
     });
   });
 });
+it("should preserve start message when no workspace name provided", () => {
+  expectParse("/new\nBuild authentication system", {
+    type: "new",
+    workspaceName: undefined,
+    trunkBranch: undefined,
+    runtime: undefined,
+    startMessage: "Build authentication system",
+  });
+});
+
+it("should preserve start message and flags when no workspace name", () => {
+  expectParse("/new -t develop\nImplement feature X", {
+    type: "new",
+    workspaceName: undefined,
+    trunkBranch: "develop",
+    runtime: undefined,
+    startMessage: "Implement feature X",
+  });
+});
+
+it("should preserve start message with runtime flag when no workspace name", () => {
+  expectParse('/new -r "ssh dev.example.com"\nDeploy to staging', {
+    type: "new",
+    workspaceName: undefined,
+    trunkBranch: undefined,
+    runtime: "ssh dev.example.com",
+    startMessage: "Deploy to staging",
+  });
+});

src/utils/slashCommands/registry.ts

@@ -539,12 +539,24 @@ const newCommandDefinition: SlashCommandDefinition = {
 
     // No workspace name provided - return undefined to open modal
     if (parsed._.length === 0) {
+      // Get trunk branch from -t flag
+      let trunkBranch: string | undefined;
+      if (parsed.t !== undefined && typeof parsed.t === "string" && parsed.t.trim().length > 0) {
+        trunkBranch = parsed.t.trim();
+      }
+
+      // Get runtime from -r flag
+      let runtime: string | undefined;
+      if (parsed.r !== undefined && typeof parsed.r === "string" && parsed.r.trim().length > 0) {
+        runtime = parsed.r.trim();
+      }
+
       return {
         type: "new",
         workspaceName: undefined,
-        trunkBranch: undefined,
-        runtime: undefined,
-        startMessage: undefined,
+        trunkBranch,
+        runtime,
+        startMessage: remainingLines,
       };
     }