fix(opencode): skip end bounds check on empty file in validateEdits (#752)

When a file has no content (lineCount === 0), any edit is a pure insert
and the end field is semantically irrelevant. The previous check
rejected payloads where end was present on an empty file because end >
lineCount always evaluated true, breaking first-call submit_plan
invocations from agents or frameworks that include end unconditionally.

- Skip end > lineCount validation when lineCount === 0; applyEdits
  handles it via splice clamping
- Add applyEdits test: edit on empty file with start=1, end=1 produces
  correct output
- Add validateEdits test: passes for empty file with start=1 and end=1

Fixes #742

Author: rcdailey

apps/opencode-plugin/index.ts

@@ -185,7 +185,11 @@ export function validateEdits(existingLines: string[], edits: PlanEdit[]): strin
       if (!Number.isInteger(edit.end) || edit.end < edit.start) {
         return `end (${edit.end}) must be >= start (${edit.start})`;
       }
-      if (edit.end > lineCount) {
+      // On an empty file (lineCount === 0) every edit is a pure insert;
+      // end is semantically meaningless and applyEdits handles it via splice
+      // clamping. Rejecting here breaks first-call payloads where the agent
+      // or framework includes end (see #742).
+      if (edit.end > lineCount && lineCount > 0) {
         return `end (${edit.end}) exceeds file length (${lineCount})`;
       }
     }

apps/opencode-plugin/submit-plan.test.ts

@@ -53,6 +53,11 @@ describe("applyEdits", () => {
     expect(result).toEqual(["hello"]);
   });
 
+  test("edit on empty file (start=1, end=1) — splice clamps gracefully (#742)", () => {
+    const result = applyEdits([], [{ start: 1, end: 1, content: "# Plan\nGoals" }]);
+    expect(result).toEqual(["# Plan", "Goals"]);
+  });
+
   test("content with trailing newline produces trailing empty string", () => {
     const result = applyEdits([], [{ start: 1, content: "line1\nline2\n" }]);
     expect(result).toEqual(["line1", "line2", ""]);
@@ -126,6 +131,12 @@ describe("validateEdits", () => {
   test("passes for empty file with start=1", () => {
     expect(validateEdits([], [{ start: 1, content: "hello" }])).toBeNull();
   });
+
+  test("passes for empty file with start=1 and end=1 (#742)", () => {
+    // Agent or framework may include end on first call; validation should
+    // not reject it since applyEdits handles this via splice clamping.
+    expect(validateEdits([], [{ start: 1, end: 1, content: "# Plan\nGoals" }])).toBeNull();
+  });
 });
 
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