Document the folded-run mail surface against @intx/mailbox

[TheGreatAxios]

CL-7276.

Analysis lane. The ticket's premise — that listFoldedMail duplicates @intx/mailbox — is wrong. The two operate on different storage planes.

folded-runs/src/mail.ts reads and writes session_mail, a Postgres table (@intx/db/schema/messages.ts:71). @intx/mailbox is a UID/MODSEQ IMAP-shaped store with no Postgres dependency at all — its deps are @intx/crypto, @intx/mime, @intx/types, arktype — and its only backings are in-memory and the sidecar's git-repo substrate store. executeSearch/executeThread take a MailboxStore, which session_mail is not.

The zero-caller signal that generated this ticket was misleading: those exports are consumed inside the vendored @intx/workflow-host, not missing from our code. Recorded as a caution for the parent audit — 'native export with zero callers' is a lead, not a finding.

Send path already composes @intx/mime natively. sendFoldedMailWithRetry has no counterpart and should stay (its hardening prevents duplicate runs on webhook sender retry). POST /workflows/:runId/mail exists but would add an authz round-trip for a row the hub already owns.

Recommends closing CL-7276. The genuine work here is CL-7103, which is a migration, not a dedup.

Docs only. No code change. Draft — not for merge.

[TheGreatAxios]

Consolidated into #531 — four independent docs files merging as one CI cycle rather than four serialized ones under strict branch protection. Content unchanged.