feat: add Sapling (sl) VCS backend support

Sapling joins git and jj as a supported VCS mode. Set `vcs = "sl"` in
config to use `hunk diff` and `hunk show` with Sapling revsets. Repos
using `.sl` or `.hg` directories are auto-detected for repo-local
config. CI workflows install Sapling for test coverage.

Author: zzl0

src/core/loaders.ts

@@ -661,6 +661,7 @@ function buildUntrackedDiffFile(
   sourcePrefix: string,
   agentContext: AgentContext | null,
 ) {
+  const absolutePath = join(repoRoot, filePath);
   const largeFileCheck = inspectLargeUntrackedFile(repoRoot, filePath);
   if (largeFileCheck.shouldSkip) {
     return buildDiffFile(
@@ -678,6 +679,40 @@ function buildUntrackedDiffFile(
     );
   }
 
+  if (input.options.vcs === "sl") {
+    if (isProbablyBinaryFile(absolutePath)) {
+      return buildDiffFile(
+        createSkippedBinaryMetadata(filePath, "new"),
+        `Binary file skipped: ${filePath}\n`,
+        index,
+        sourcePrefix,
+        agentContext,
+        { isBinary: true, isUntracked: true },
+      );
+    }
+
+    const patch = createTwoFilesPatch(
+      "/dev/null",
+      escapeUntrackedPatchPath(filePath),
+      "",
+      fs.readFileSync(absolutePath, "utf8"),
+      "",
+      "",
+      { context: 3 },
+    ).replaceAll("\r\n", "\n");
+
+    return buildDiffFile(
+      parseUntrackedPatchFile(patch, filePath),
+      patch,
+      index,
+      sourcePrefix,
+      agentContext,
+      {
+        isUntracked: true,
+      },
+    );
+  }
+
   const patch = normalizeUntrackedPatchHeaders(
     runGitUntrackedFileDiffText(input, filePath, { repoRoot }),
     filePath,

src/core/sl.test.ts

@@ -0,0 +1,102 @@
+import { afterEach, describe, expect, test } from "bun:test";
+import { mkdtempSync, realpathSync, rmSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { buildSlDiffArgs, runSlText } from "./sl";
+
+const tempDirs: string[] = [];
+
+function cleanupTempDirs() {
+  while (tempDirs.length > 0) {
+    const dir = tempDirs.pop();
+    if (dir) {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  }
+}
+
+function createTempDir(prefix: string) {
+  const dir = realpathSync(mkdtempSync(join(tmpdir(), prefix)));
+  tempDirs.push(dir);
+  return dir;
+}
+
+function sl(cwd: string, ...cmd: string[]) {
+  const proc = Bun.spawnSync(["sl", "--noninteractive", "--color", "never", ...cmd], {
+    cwd,
+    stdout: "pipe",
+    stderr: "pipe",
+    stdin: "ignore",
+  });
+
+  if (proc.exitCode !== 0) {
+    const stderr = Buffer.from(proc.stderr).toString("utf8");
+    throw new Error(stderr.trim() || `sl ${cmd.join(" ")} failed`);
+  }
+
+  return Buffer.from(proc.stdout).toString("utf8");
+}
+
+function createTempSlRepo(prefix: string) {
+  const dir = createTempDir(prefix);
+
+  sl(dir, "init", "--git");
+
+  return dir;
+}
+
+afterEach(() => {
+  cleanupTempDirs();
+});
+
+describe("sl command helpers", () => {
+  test("reports a friendly error when sl is not installed or not on PATH", () => {
+    expect(() =>
+      runSlText({
+        input: {
+          kind: "vcs",
+          staged: false,
+          options: { mode: "auto", vcs: "sl" },
+        },
+        args: ["root"],
+        slExecutable: "definitely-not-a-real-sl-binary",
+      }),
+    ).toThrow(
+      'Sapling is required for `hunk diff` when `vcs = "sl"`, but `definitely-not-a-real-sl-binary` was not found in PATH.',
+    );
+  });
+
+  test("reports a friendly error outside a sl repository", () => {
+    const dir = createTempDir("hunk-sl-nonrepo-");
+
+    expect(() =>
+      runSlText({
+        input: {
+          kind: "vcs",
+          staged: false,
+          options: { mode: "auto", vcs: "sl" },
+        },
+        args: ["root"],
+        cwd: dir,
+      }),
+    ).toThrow('`hunk diff` must be run inside a Sapling repository when `vcs = "sl"`.');
+  });
+
+  test("reports a friendly error for invalid revsets", () => {
+    const dir = createTempSlRepo("hunk-sl-invalid-revset-");
+    const input = {
+      kind: "vcs" as const,
+      range: "missing_revision",
+      staged: false,
+      options: { mode: "auto" as const, vcs: "sl" as const },
+    };
+
+    expect(() =>
+      runSlText({
+        input,
+        args: buildSlDiffArgs(input),
+        cwd: dir,
+      }),
+    ).toThrow("`hunk diff missing_revision` could not resolve Sapling revset `missing_revision`.");
+  });
+});