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
Add markdown mirror generation for DB graphs, including page and journal paths, regeneration, debounced write handling, and settings UI.
Serialize page and block property values into mirrored Markdown by resolving property-value entities to their display content.
Persist mirror files through the node worker platform and make browser worker mirror storage fail fast as unsupported.
Fix Electron userAppCfgs writes to avoid returning unserializable Electron state over IPC.
Add ADR and targeted tests for mirror generation, worker wiring, platform storage, and graph path handling.
