docs(refactor-module-logic): Clarify FR-003 in spec

docs(refactor-module-logic): Clarify FR-003 in spec Updated Functional Requirement FR-003 in the refactoring specification (003-refactor-module-logic) to explicitly state that the 'logseq_converter.obsidian' module must solely handle the conversion of Logseq model objects to Obsidian-flavored Markdown files and their writing to the filesystem. This provides a clearer definition of the module's responsibility. The associated checklist has also been updated to reflect this change.