cairn/changes/maildir-custom-keywords/delta.md
The Maildir backend SHALL surface custom (non-IANA) keywords on read when told which convention the mailbox uses, so a keyword written by dovecot, mbsync, OfflineIMAP, mutt or notmuch matches a flag <name> search as it does on the network backends. maildir.keywords.dovecot SHALL resolve the lowercase info-section slot letters through the resolved mailbox's own dovecot-keywords file, and maildir.keywords.header SHALL read keywords from X-Keywords (comma-separated) or X-Label (space-separated). Both default to off, and with both off the flag set SHALL be exactly the six standard info-section letters as before.
Keyword reading is not a round trip: no command can name a custom keyword, so a FlagOp::Set store SHALL replace the whole set and drop any keyword the message carried.
A sidecar that is absent, unreadable or disabled SHALL yield no keywords rather than fail the listing, since a mailbox without one is the normal case rather than an error.
Maildir, m2dir and pimdir SHALL adapt io-maildir, io-m2dir and io-pimdir. Maildir stores added messages under cur/ and SHALL read an entry's flags through io-maildir rather than parsing the filename itself, so the meaning of a Maildir name is decided in the library that owns the format. m2dir is content-addressed with no native copy or move, so those are a get plus a store (plus a delete for move), and its flags live in a .meta/<id>.flags sidecar. m2dir mailbox rename and message copy/move remain unavailable until io-m2dir supports them. pimdir is an offline cache the sync engine (io-replica + io-pimdir) populates: reads project the store's shared items (io-pimdir's client read API) from the stored v: 1 meta without body reads, and writes are staged io-replica mutate mutations a later sync propagates rather than direct SQL.