feat(ce-review): add base: and plan: arguments, extract scope detection (#405)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/review-skill-contract.test.ts

@@ -173,9 +173,21 @@ describe("ce-review contract", () => {
     expect(content).not.toContain("git diff -U10 HEAD")
     expect(content).not.toContain("git diff --cached")
 
-    // All three scope paths must emit ERROR when BASE is unresolved
-    const errorMatches = content.match(/echo "ERROR: Unable to resolve/g)
-    expect(errorMatches?.length).toBe(3) // PR mode, branch mode, standalone mode
+    // PR mode still has an inline error for unresolved base
+    expect(content).toContain('echo "ERROR: Unable to resolve PR base branch')
+
+    // Branch and standalone modes delegate to resolve-base.sh and check its ERROR: output.
+    // The script itself emits ERROR: when the base is unresolved.
+    expect(content).toContain("references/resolve-base.sh")
+    const resolveScript = await readRepoFile(
+      "plugins/compound-engineering/skills/ce-review/references/resolve-base.sh",
+    )
+    expect(resolveScript).toContain("ERROR:")
+
+    // Branch and standalone modes must stop on script error, not fall back
+    expect(content).toContain(
+      "If the script outputs an error, stop instead of falling back to `git diff HEAD`",
+    )
   })
 
   test("orchestration callers pass explicit mode flags", async () => {