feat: Complete Issue #41 - Add TOML frontmatter support

- Enhanced frontmatter parser to detect and parse TOML format
- Added TOML format detection heuristics before YAML parsing
- Created TOML test fixture with nested sections
- Fixed parsing order to prevent TOML-to-string conversion
- All frontmatter formats (YAML, JSON, TOML) now fully supported
- Validated all acceptance criteria for Issue #41

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

Co-Authored-By: Claude <noreply@anthropic.com>

markitect/matter_frontmatter/parser.py

@@ -5,6 +5,7 @@ Frontmatter parser for extracting and manipulating YAML/JSON/TOML frontmatter.
 import re
 import yaml
 import json
+import toml
 from typing import Dict, Any, List, Optional
 from .stats import FrontmatterStats
 
@@ -27,20 +28,32 @@ class FrontmatterParser:
         if not frontmatter_content:
             return {}
 
-        # Try to parse as YAML first (most common)
+        # Try to detect format first for better parsing
+        content = frontmatter_content.strip()
+
+        # Try TOML first if it looks like TOML
+        if '=' in content and ('[' in content or '"' in content):
+            try:
+                return toml.loads(frontmatter_content)
+            except toml.TomlDecodeError:
+                pass
+
+        # Try JSON if it looks like JSON
+        if content.startswith('{') and content.endswith('}'):
+            try:
+                return json.loads(frontmatter_content)
+            except json.JSONDecodeError:
+                pass
+
+        # Default to YAML (most common)
         try:
-            return yaml.safe_load(frontmatter_content) or {}
+            result = yaml.safe_load(frontmatter_content)
+            # Ensure we got a dictionary, not a string
+            if isinstance(result, dict):
+                return result
         except yaml.YAMLError:
             pass
 
-        # Try to parse as JSON
-        try:
-            return json.loads(frontmatter_content)
-        except json.JSONDecodeError:
-            pass
-
-        # TODO: Add TOML support in future iterations
-
         return {}
 
     def set_frontmatter_value(self, text: str, key: str, value: Any) -> str:
@@ -118,7 +131,7 @@ class FrontmatterParser:
         """
         frontmatter = self.extract_frontmatter(text)
 
-        if not frontmatter:
+        if not frontmatter or not isinstance(frontmatter, dict):
             return FrontmatterStats(
                 has_frontmatter=False,
                 total_fields=0,
@@ -166,8 +179,11 @@ class FrontmatterParser:
         content = content.strip()
         if content.startswith('{') and content.endswith('}'):
             return "json"
+        elif '=' in content and '[' in content:
+            # Simple heuristic for TOML
+            return "toml"
         else:
-            # Default to YAML for now
+            # Default to YAML
             return "yaml"
 
     def _set_nested_value(self, data: Dict[str, Any], key: str, value: Any) -> None:

tests/fixtures/frontmatter_test_files/toml_frontmatter.md

@@ -0,0 +1,29 @@
+---
+title = "TOML Frontmatter Test Document"
+author = "Test Author"
+date = "2025-10-02"
+tags = ["toml", "frontmatter", "testing"]
+version = 1.4
+published = true
+
+[config]
+theme = "light"
+language = "en"
+debug = false
+
+[features]
+search = true
+toc = true
+navigation = true
+---
+
+# TOML Frontmatter Test Document
+
+This document uses TOML format for frontmatter instead of YAML or JSON.
+
+The frontmatter parser should handle TOML format correctly and extract values like:
+- title: "TOML Frontmatter Test Document"
+- config.theme: "light"
+- features.search: true
+
+This tests the parser's ability to handle all three frontmatter formats as required by Issue #41.