chore(mcp): convert lib/local-branch.js to TypeScript (Phase 2 of #7291)

test/fixtures/local-scorer/scorer-json-array-stderr-then-sigkill.mjs

@@ -0,0 +1,2 @@
+process.stderr.write("[1,2,3]\n");
+process.kill(process.pid, "SIGKILL");

test/fixtures/local-scorer/scorer-json-number-stderr-then-sigkill.mjs

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+process.stderr.write("42\n");
+process.kill(process.pid, "SIGKILL");

test/fixtures/local-scorer/scorer-missing-scores.mjs

@@ -0,0 +1 @@
+process.stdout.write(JSON.stringify({ activeModel: "some-model" }) + "\n");

test/fixtures/local-scorer/scorer-non-object-json.mjs

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+process.stdout.write("42\n");

test/fixtures/local-scorer/scorer-scored-json-then-fail.mjs

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+process.stdout.write(JSON.stringify({ sourceTokenScore: 10, totalTokenScore: 20 }) + "\n");
+process.exit(1);

test/fixtures/local-scorer/scorer-scoreless-json-then-fail.mjs

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+process.stdout.write(JSON.stringify({}) + "\n");
+process.exit(1);

test/fixtures/local-scorer/scorer-stderr-then-fail.mjs

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+process.stderr.write(`crash detail: ${"x".repeat(200)}\n`);
+process.exit(2);

test/fixtures/local-scorer/scorer-stderr-then-sigkill.mjs

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+process.stderr.write("dying before cleanup\n");
+process.kill(process.pid, "SIGKILL");

test/fixtures/local-scorer/scorer-stdout-then-fail.mjs

@@ -0,0 +1,2 @@
+process.stdout.write("partial junk output before crashing\n");
+process.exit(3);