Move the Logseq Markdown Mirror syntax/export work out of the two-way sync branch while leaving file-to-DB sync behavior behind.
Details:\n- add docs/logseq-markdown-syntax.md and update ADR 0016 with the one-way mirror syntax contract\n- export mirror files with page id markers, property list items, nested default property values, node page refs, task status markers, and datetime values with time\n- preserve block refs as uuid links where needed while keeping page/node refs readable\n- update focused export and markdown mirror tests
Validation:\n- bb dev:lint-and-test
