feat(extensions): support .extensionignore to exclude files during install (#1781)

* feat(extensions): support .extensionignore to exclude files during install

Add .extensionignore support so extension authors can exclude files and
folders from being copied when users run 'specify extension add'.

The file uses glob-style patterns (one per line), supports comments (#),
blank lines, trailing-slash directory patterns, and relative path matching.
The .extensionignore file itself is always excluded from the copy.

- Add _load_extensionignore() to ExtensionManager
- Integrate ignore function into shutil.copytree in install_from_directory
- Document .extensionignore in EXTENSION-DEVELOPMENT-GUIDE.md
- Add 6 tests covering all pattern matching scenarios
- Bump version to 0.1.14

* fix(extensions): use pathspec for gitignore-compatible .extensionignore matching

Replace fnmatch with pathspec.GitIgnoreSpec to get proper .gitignore
semantics where * does not cross directory boundaries. This addresses
review feedback on #1781.

Changes:
- Switch from fnmatch to pathspec>=0.12.0 (GitIgnoreSpec.from_lines)
- Normalize backslashes in patterns for cross-platform compatibility
- Distinguish directories from files for trailing-slash patterns
- Update docs to accurately describe supported pattern semantics
- Add edge-case tests: .., absolute paths, empty file, backslashes,
  * vs ** boundary behavior, and ! negation
- Move changelog entry to [Unreleased] section

src/specify_cli/extensions.py

@@ -14,10 +14,12 @@ import zipfile
 import shutil
 from dataclasses import dataclass
 from pathlib import Path
-from typing import Optional, Dict, List, Any
+from typing import Optional, Dict, List, Any, Callable, Set
 from datetime import datetime, timezone
 import re
 
+import pathspec
+
 import yaml
 from packaging import version as pkg_version
 from packaging.specifiers import SpecifierSet, InvalidSpecifier
@@ -280,6 +282,70 @@ class ExtensionManager:
         self.extensions_dir = project_root / ".specify" / "extensions"
         self.registry = ExtensionRegistry(self.extensions_dir)
 
+    @staticmethod
+    def _load_extensionignore(source_dir: Path) -> Optional[Callable[[str, List[str]], Set[str]]]:
+        """Load .extensionignore and return an ignore function for shutil.copytree.
+
+        The .extensionignore file uses .gitignore-compatible patterns (one per line).
+        Lines starting with '#' are comments. Blank lines are ignored.
+        The .extensionignore file itself is always excluded.
+
+        Pattern semantics mirror .gitignore:
+        - '*' matches anything except '/'
+        - '**' matches zero or more directories
+        - '?' matches any single character except '/'
+        - Trailing '/' restricts a pattern to directories only
+        - Patterns with '/' (other than trailing) are anchored to the root
+        - '!' negates a previously excluded pattern
+
+        Args:
+            source_dir: Path to the extension source directory
+
+        Returns:
+            An ignore function compatible with shutil.copytree, or None
+            if no .extensionignore file exists.
+        """
+        ignore_file = source_dir / ".extensionignore"
+        if not ignore_file.exists():
+            return None
+
+        lines: List[str] = ignore_file.read_text().splitlines()
+
+        # Normalise backslashes in patterns so Windows-authored files work
+        normalised: List[str] = []
+        for line in lines:
+            stripped = line.strip()
+            if stripped and not stripped.startswith("#"):
+                normalised.append(stripped.replace("\\", "/"))
+            else:
+                # Preserve blanks/comments so pathspec line numbers stay stable
+                normalised.append(line)
+
+        # Always ignore the .extensionignore file itself
+        normalised.append(".extensionignore")
+
+        spec = pathspec.GitIgnoreSpec.from_lines(normalised)
+
+        def _ignore(directory: str, entries: List[str]) -> Set[str]:
+            ignored: Set[str] = set()
+            rel_dir = Path(directory).relative_to(source_dir)
+            for entry in entries:
+                rel_path = str(rel_dir / entry) if str(rel_dir) != "." else entry
+                # Normalise to forward slashes for consistent matching
+                rel_path_fwd = rel_path.replace("\\", "/")
+
+                entry_full = Path(directory) / entry
+                if entry_full.is_dir():
+                    # Append '/' so directory-only patterns (e.g. tests/) match
+                    if spec.match_file(rel_path_fwd + "/"):
+                        ignored.add(entry)
+                else:
+                    if spec.match_file(rel_path_fwd):
+                        ignored.add(entry)
+            return ignored
+
+        return _ignore
+
     def check_compatibility(
         self,
         manifest: ExtensionManifest,
@@ -353,7 +419,8 @@ class ExtensionManager:
         if dest_dir.exists():
             shutil.rmtree(dest_dir)
 
-        shutil.copytree(source_dir, dest_dir)
+        ignore_fn = self._load_extensionignore(source_dir)
+        shutil.copytree(source_dir, dest_dir, ignore=ignore_fn)
 
         # Register commands with AI agents
         registered_commands = {}