feat: resolve Git LFS pointers via the raw URL endpoint

Files tracked by Git LFS used to come out as the pointer text:

    version https://git-lfs.github.com/spec/v1
    oid sha256:...
    size ...

…because GitHub's blob API returns the pointer, not the resolved
content. Detect that prefix on the first ~150 bytes of the blob stream
and switch to a fresh fetch via the web raw URL
(github.com/<owner>/<repo>/raw/<commit>/<path>), which auto-redirects
to media.githubusercontent.com and resolves the LFS object — auth
header carries through. Non-LFS files are forwarded through the
existing pipeline unchanged.

Fixes #95.

test/lfs-detect.test.js

@@ -0,0 +1,42 @@
+const { expect } = require("chai");
+const { Readable } = require("stream");
+
+// Standalone test of the LFS-pointer detection shape used in
+// GitHubStream#resolveLfsPointer. We can't easily import that method (it's
+// private and the file pulls in heavy GitHub plumbing), so this mirrors the
+// detection logic to confirm the head-bytes check.
+
+const LFS_PREFIX = "version https://git-lfs.github.com/spec/";
+
+function isLfsPointer(buf) {
+  return (
+    buf.length >= LFS_PREFIX.length &&
+    buf.toString("utf8", 0, LFS_PREFIX.length) === LFS_PREFIX
+  );
+}
+
+describe("LFS pointer detection (#95)", function () {
+  it("recognizes the standard pointer prefix", function () {
+    const pointer = Buffer.from(
+      "version https://git-lfs.github.com/spec/v1\n" +
+        "oid sha256:abc123\nsize 12345\n"
+    );
+    expect(isLfsPointer(pointer)).to.equal(true);
+  });
+
+  it("doesn't false-positive on plain text starting with 'version'", function () {
+    const fake = Buffer.from(
+      "version 1.2.3\nA short release notes file mentioning git-lfs.\n"
+    );
+    expect(isLfsPointer(fake)).to.equal(false);
+  });
+
+  it("doesn't false-positive on binary headers", function () {
+    const elf = Buffer.from([0x7f, 0x45, 0x4c, 0x46, ...new Array(100).fill(0)]);
+    expect(isLfsPointer(elf)).to.equal(false);
+  });
+
+  it("handles short streams below the prefix length", function () {
+    expect(isLfsPointer(Buffer.from("vers"))).to.equal(false);
+  });
+});