feat: merge ThreeTxtFileHelper into URLNotesGrabberCORE

Folds the standalone ThreeTxtFileHelper tool into URLNotesGrabberCORE so
text-file ingest/output/correct lives alongside the API scraper. Adds
new flags -ingest, -output, -correct (with -apply), -updatepaths, and a
one-time -importposts <posts.db> migration.

Schema: Blogs.TTFolderPath and Posts.PostType are added by an idempotent
migration. On (BlogName, PostID) collisions, content columns are
overwritten while engagement columns (ByLikes, RootBlogName, RootURL,
HasNotesGathered, NotFound, NotesGatheredDateTime, Likes*) are preserved.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

URLNotesGrabberCORE/prefixes.txt

@@ -0,0 +1,20 @@
+Post ID
+reblog URL
+Date
+Has Image
+Post URL
+Slug
+Reblog Key
+Reblog Name
+Summary
+Quote
+Body
+Tags
+Link
+Photo URL
+Photo Caption
+Downloaded Files
+Audio Caption
+Question
+Answer
+Title