refactor: eliminate isinstance() checks with pure polymorphism

Replace all isinstance() type checking with proper polymorphic methods:

Pure Polymorphism Methods:
- get_summary_info(): Each class returns its own summary format
- is_single_file(): Boolean check without isinstance()
- gather_contents(): Recursive content gathering via method dispatch
- get_display_name(): Tree display formatting (/, -> target, etc.)
- has_children(): Check for child nodes without type checking

Benefits:
- No isinstance() 'clochard' style code anywhere
- True duck typing - just call methods and let Python dispatch
- Cleaner, more maintainable code
- Each class encapsulates its own behavior
- Easy to extend with new node types

Code Changes:
- FileSystemNode: Base implementations for all methods
- FileSystemFile: is_single_file()=True, summary with line count
- FileSystemDirectory: get_display_name() adds '/', has_children() checks list
- FileSystemSymlink: get_display_name() shows '-> target'
- output_formatter.py: Use polymorphic methods instead of isinstance()

This is proper OOP - objects know their own behavior!

Author: cursoragent

src/gitingest/output_formatter.py

@@ -41,14 +41,9 @@ def format_node(node: FileSystemNode, query: IngestionQuery) -> tuple[str, str,
         A tuple containing the summary, directory structure, and file contents.
 
     """
-    is_single_file = isinstance(node, FileSystemFile)
+    is_single_file = node.is_single_file()
     summary = _create_summary_prefix(query, single_file=is_single_file)
-
-    if isinstance(node, FileSystemDirectory):
-        summary += f"Files analyzed: {node.file_count}\n"
-    elif isinstance(node, FileSystemFile):
-        summary += f"File: {node.name}\n"
-        summary += f"Lines: {len(node.content.splitlines()):,}\n"
+    summary += node.get_summary_info()
 
     tree = "Directory structure:\n" + _create_tree_structure(query, node=node)
 
@@ -118,11 +113,7 @@ def _gather_file_contents(node: FileSystemNode) -> str:
         The concatenated content of all files under the given node.
 
     """
-    if not isinstance(node, FileSystemDirectory):
-        return node.content_string
-
-    # Recursively gather contents of all files under the current directory
-    return "\n".join(_gather_file_contents(child) for child in node.children)
+    return node.gather_contents()
 
 
 def _create_tree_structure(
@@ -161,16 +152,11 @@ def _create_tree_structure(
     tree_str = ""
     current_prefix = "└── " if is_last else "├── "
 
-    # Indicate directories with a trailing slash
-    display_name = node.name
-    if isinstance(node, FileSystemDirectory):
-        display_name += "/"
-    elif isinstance(node, FileSystemSymlink):
-        display_name += " -> " + node.target
-
+    # Get the display name (handles directory slash, symlink target, etc.)
+    display_name = node.get_display_name()
     tree_str += f"{prefix}{current_prefix}{display_name}\n"
 
-    if isinstance(node, FileSystemDirectory) and node.children:
+    if node.has_children():
         prefix += "    " if is_last else "│   "
         for i, child in enumerate(node.children):
             tree_str += _create_tree_structure(query, node=child, prefix=prefix, is_last=i == len(node.children) - 1)

src/gitingest/schemas/filesystem.py

@@ -96,6 +96,26 @@ def get_content(self) -> str:
         except Exception as e:
             return f"Error reading content of {self.name}: {e}"
 
+    def get_summary_info(self) -> str:
+        """Return summary information. Override in subclasses."""
+        return ""
+
+    def is_single_file(self) -> bool:
+        """Return whether this node represents a single file."""
+        return False
+
+    def gather_contents(self) -> str:
+        """Gather file contents. Override in subclasses."""
+        return self.content_string
+
+    def get_display_name(self) -> str:
+        """Get display name for tree view. Override in subclasses."""
+        return self.name
+
+    def has_children(self) -> bool:
+        """Return whether this node has children to display."""
+        return False
+
     @property
     def content(self) -> str:
         """Return file content (simplified version for backward compatibility)."""
@@ -110,6 +130,14 @@ def get_sort_priority(self) -> int:
         """Files have priority 0 for sorting."""
         return 0
 
+    def get_summary_info(self) -> str:
+        """Return file summary information."""
+        return f"File: {self.name}\nLines: {len(self.content.splitlines()):,}\n"
+
+    def is_single_file(self) -> bool:
+        """Files are single files."""
+        return True
+
     def render_tree(self, prefix: str = "", *, is_last: bool = True) -> list[str]:
         """Render the tree representation of this file."""
         current_prefix = "└── " if is_last else "├── "
@@ -127,6 +155,22 @@ def get_content(self) -> str:
         msg = "Cannot read content of a directory node"
         raise ValueError(msg)
 
+    def get_summary_info(self) -> str:
+        """Return directory summary information."""
+        return f"Files analyzed: {self.file_count}\n"
+
+    def gather_contents(self) -> str:
+        """Recursively gather contents of all files under this directory."""
+        return "\n".join(child.gather_contents() for child in self.children)
+
+    def get_display_name(self) -> str:
+        """Directories get a trailing slash."""
+        return self.name + "/"
+
+    def has_children(self) -> bool:
+        """Directories have children if the list is not empty."""
+        return bool(self.children)
+
     def render_tree(self, prefix: str = "", *, is_last: bool = True) -> list[str]:
         """Render the tree representation of this directory."""
         lines = []
@@ -178,6 +222,10 @@ def get_content(self) -> str:
         """Symlinks content is what they point to."""
         return self.target
 
+    def get_display_name(self) -> str:
+        """Symlinks show target."""
+        return f"{self.name} -> {self.target}"
+
     def render_tree(self, prefix: str = "", *, is_last: bool = True) -> list[str]:
         """Render the tree representation of this symlink."""
         current_prefix = "└── " if is_last else "├── "