fix: handle UTF-8 BOM in frontmatter parsing

Fixes #452 - Imported conversations not fully indexed

Files with UTF-8 BOM (Byte Order Mark) at the start would fail frontmatter
detection, causing:
- Title to fall back to filename instead of frontmatter value
- Permalink to be null in the database

Added strip_bom() helper function and updated all frontmatter-related
functions to strip BOM before processing:
- has_frontmatter()
- parse_frontmatter()
- remove_frontmatter()
- EntityParser.parse_markdown_content()

Added comprehensive tests for BOM handling with various scenarios.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

src/basic_memory/file_utils.py

@@ -69,6 +69,28 @@ async def compute_checksum(content: Union[str, bytes]) -> str:
         raise FileError(f"Failed to compute checksum: {e}")
 
 
+# UTF-8 BOM character that can appear at the start of files
+UTF8_BOM = '\ufeff'
+
+
+def strip_bom(content: str) -> str:
+    """Strip UTF-8 BOM from the start of content if present.
+
+    BOM (Byte Order Mark) characters can be present in files created on Windows
+    or copied from certain sources. They should be stripped before processing
+    frontmatter. See issue #452.
+
+    Args:
+        content: Content that may start with BOM
+
+    Returns:
+        Content with BOM removed if present
+    """
+    if content and content.startswith(UTF8_BOM):
+        return content[1:]
+    return content
+
+
 async def write_file_atomic(path: FilePath, content: str) -> None:
     """
     Write file with atomic operation using temporary file.
@@ -113,7 +135,8 @@ def has_frontmatter(content: str) -> bool:
     if not content:
         return False
 
-    content = content.strip()
+    # Strip BOM before checking for frontmatter markers
+    content = strip_bom(content).strip()
     if not content.startswith("---"):
         return False
 
@@ -134,6 +157,8 @@ def parse_frontmatter(content: str) -> Dict[str, Any]:
         ParseError: If frontmatter is invalid or parsing fails
     """
     try:
+        # Strip BOM before parsing frontmatter
+        content = strip_bom(content)
         if not content.strip().startswith("---"):
             raise ParseError("Content has no frontmatter")
 
@@ -175,7 +200,8 @@ def remove_frontmatter(content: str) -> str:
     Raises:
         ParseError: If content starts with frontmatter marker but is malformed
     """
-    content = content.strip()
+    # Strip BOM before processing
+    content = strip_bom(content).strip()
 
     # Return as-is if no frontmatter marker
     if not content.startswith("---"):

src/basic_memory/markdown/entity_parser.py

@@ -227,6 +227,11 @@ class EntityParser:
         Returns:
             EntityMarkdown with parsed content
         """
+        # Strip BOM before parsing (can be present in files from Windows or certain sources)
+        # See issue #452
+        from basic_memory.file_utils import strip_bom
+        content = strip_bom(content)
+
         # Parse frontmatter with proper error handling for malformed YAML
         try:
             post = frontmatter.loads(content)