Replace fragile parent traversal with marker-based project root detection

[Copilot]

The _get_default_schema_path() method used hardcoded parent.parent.parent traversal to find the project root, breaking when directory structure changes.

Changes

Robust project root detection:

• Search upward for pyproject.toml marker file (primary method)
• Fallback to D4D_PROJECT_ROOT environment variable
• Max depth limit (30 levels) prevents runaway searches

Improved error handling:

• List both attempted schema paths (data_sheets_schema_all.yaml and data_sheets_schema.yaml)
• Include starting directory in error messages
• Clear guidance on using environment variable fallback

Code quality:

• Module-level imports (os moved from method scope)
• Use os.getenv() per Python conventions
• Clarifying comments for filesystem root detection

Example

# Before: Fragile and breaks if structure changes
project_root = self.prompts_dir.parent.parent.parent

# After: Robust marker-based search
project_root = self._find_project_root(self.prompts_dir, marker_file="pyproject.toml")
if project_root is None:
    env_root = os.getenv('D4D_PROJECT_ROOT')
    # ... fallback logic

The new _find_project_root() method walks up the directory tree until finding pyproject.toml, working from any nesting depth.

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