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/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)