feat: change document path to /pub/{rkey}/{slug}
Replace path-based document addressing with a stable canonical path
tied to the ATProto record key. New records are created with a
placeholder path, then immediately updated via putRecord with the
final /pub/{rkey}/{slug} path.
- Remove pathPrefix and pathTemplate config options
- Remove resolvePostPath() and resolvePathTemplate() from markdown.ts
- createDocument() always makes 2 API calls (create + putRecord)
- updateDocument() builds path from parsed rkey
- sync and publish commands switch to atUri-based record matching