feat: add path validation for commit_files MCP tool (#796)

Add validatePathWithinRepo helper to ensure file paths resolve within the repository root directory. This hardens the commit_files tool by validating paths before file operations.

Changes:
- Add src/mcp/path-validation.ts with async path validation using realpath
- Update commit_files to validate all paths before reading files
- Prevent symlink-based path escapes by resolving real paths
- Add comprehensive test coverage including symlink attack scenarios

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

src/mcp/path-validation.ts

@@ -0,0 +1,64 @@
+import { realpath } from "fs/promises";
+import { resolve, sep } from "path";
+
+/**
+ * Validates that a file path resolves within the repository root.
+ * Prevents path traversal attacks via "../" sequences and symlinks.
+ * @param filePath - The file path to validate (can be relative or absolute)
+ * @param repoRoot - The repository root directory
+ * @returns The resolved absolute path (with symlinks resolved) if valid
+ * @throws Error if the path resolves outside the repository root
+ */
+export async function validatePathWithinRepo(
+  filePath: string,
+  repoRoot: string,
+): Promise<string> {
+  // First resolve the path string (handles .. and . segments)
+  const initialPath = resolve(repoRoot, filePath);
+
+  // Resolve symlinks to get the real path
+  // This prevents symlink attacks where a link inside the repo points outside
+  let resolvedRoot: string;
+  let resolvedPath: string;
+
+  try {
+    resolvedRoot = await realpath(repoRoot);
+  } catch {
+    throw new Error(`Repository root '${repoRoot}' does not exist`);
+  }
+
+  try {
+    resolvedPath = await realpath(initialPath);
+  } catch {
+    // File doesn't exist yet - fall back to checking the parent directory
+    // This handles the case where we're creating a new file
+    const parentDir = resolve(initialPath, "..");
+    try {
+      const resolvedParent = await realpath(parentDir);
+      if (
+        resolvedParent !== resolvedRoot &&
+        !resolvedParent.startsWith(resolvedRoot + sep)
+      ) {
+        throw new Error(
+          `Path '${filePath}' resolves outside the repository root`,
+        );
+      }
+      // Parent is valid, return the initial path since file doesn't exist yet
+      return initialPath;
+    } catch {
+      throw new Error(
+        `Path '${filePath}' resolves outside the repository root`,
+      );
+    }
+  }
+
+  // Path must be within repo root (or be the root itself)
+  if (
+    resolvedPath !== resolvedRoot &&
+    !resolvedPath.startsWith(resolvedRoot + sep)
+  ) {
+    throw new Error(`Path '${filePath}' resolves outside the repository root`);
+  }
+
+  return resolvedPath;
+}