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/syncignore.rs

@@ -1,4 +1,4 @@
-//! Selective-sync ignore rules for mirror folders (features.md 1.1).
+//! Selective-sync ignore rules for mirror folders.
 //!
 //! A mirror folder otherwise uploads every path beneath its root, which for a
 //! project directory means `node_modules/`, `.git/`, and `target/` — quota