chore: cite docs/BUGS.md and drop references to notes outside the repo

66 comments cited `bugs.md B<n>`, the ledger's old name; they now say
`docs/BUGS.md B<n>`, and every cited number has an entry. About 90 more
cited offline.md, devices.md, features.md, mount-architecture.md,
plan.md and improvements.md, planning notes that were never in the
repository. Those citations are removed; the comments keep their text.

crates/pdfs-core/src/db/trash.rs

@@ -102,5 +102,5 @@ impl Db {
         Ok(seen)
     }
 
-    // ---- device sync (devices.md) -----------------------------------------
+    // ---- device sync -----------------------------------------
 }