Knowledge base "Reindex" now re-reads the original source

What changed

Reindexing a knowledge base data source now re-acquires it from its real source before rebuilding the search index, instead of rebuilding from the copy the knowledge base already holds:

• File — the user's original file is copied over the knowledge base's copy again; a file that goes through a document processor is processed again.
• Folder — rescanned from the original folder (unchanged).
• URL — the page is fetched again and the stored snapshot is replaced.
• Note — the snapshot is rewritten from the note text stored in the app.

Reindexing a file or folder whose original has been moved or deleted is now rejected with "Cannot reindex a knowledge item whose source file or folder no longer exists; delete it and add it again to rebuild". Previously it succeeded by rebuilding from the internal copy.

Why this matters to the user

Reindex finally means "pick up my edits". Editing a document in your own editor and clicking Reindex now updates search results — previously it changed nothing, because only the app's private copy was ever read.

Two visible consequences:

• Reindex fails for an item whose original file or folder is gone. This includes items migrated from v1, whose recorded original path points into the v1 data directory — if that directory was cleaned up, or the profile was restored across platforms, those items can no longer be reindexed.
• Reindexing a URL whose page is now dead (404, domain gone, login required) fails, and that item loses its index until the page is reachable again.

What the user should do

If reindex reports the source no longer exists, delete the data source and add it again.

To maintain a document outside the app and have reindex pick up the edits, add it as a file data source pointing at your own file. Editing the raw/*.md files inside a knowledge base directory is not a supported workflow — reindex overwrites them.

Notes for release manager

Pairs with the fix for the note preview / search inconsistency reported in #19492: a note's text in the app is authoritative, and its snapshot file is now regenerated from it on every reindex, so the preview panel and search results can no longer disagree.