Add mavmaild, the read-only IMAP poller that feeds mail intake (#246)

The extraction seam landed on the previous branch but nothing fed it. This
adds the daemon that does: every interval it opens one mailbox read-only
(EXAMINE + BODY.PEEK, so reading leaves no \Seen behind), fetches the UIDs
it has not handed over yet, and posts each message to core over
ingest_mail. Core runs the model and writes task candidates; this daemon
writes nothing and cannot create a reminder.

It is a separate daemon because of the credential. mavpoll set the
precedent with the zenmoney token (#125): the module talking to the third
party holds the secret, reads it from a file so it never lands in argv, in
docker-compose.yml or in shell history, and core never sees it. There is
deliberately no -password flag, and a test asserts that.

Off unless configured at both ends: without -password-file the daemon
refuses to start, and if core has no email block the first ingest returns
ErrUnknownMethod, which disables the reader instead of hammering a socket
that will keep refusing. A seen-UID state file (0600, atomic write) keeps a
restart from re-extracting the whole lookback window; correctness does not
depend on it, since capture dedupes on normalised text. Logs are counts and
UIDs — no subject, sender or body.

Verified with an in-process IMAP server and a fake core: bulk mail is
filtered before core is asked, seen UIDs are not re-fetched, a failed
ingest is retried next poll, ErrUnknownMethod stops at the first message,
and state survives a restart. The live half is untested by design — no IMAP
credential exists on this box; setup is written up as QA steps.

Vikunja #246

CLAUDE.md

@@ -34,7 +34,7 @@ CGO daemons (`mavend`, `mavsttd`, `mavttsd`, `mavenclient`) need the vendored to
 and libs wired through the Makefile — **do not** call `go build` on them bare, use `make`:
 
 ```sh
-make build          # all 8 binaries
+make build          # all 9 binaries
 make build-web      # single daemon (pure-Go ones: web/waked/poll/caldav build without CGO)
 make test           # go test -race across ./internal/... ./cmd/... with CGO env set
 ```
@@ -62,6 +62,7 @@ Pure-Go packages (`router`, `memory`, `mavweb`, …) run under a plain `go test
 | `mavenclient` | Voice loop client (mic → stt → core → tts). |
 | `mavpoll` | Telegram long-poll reach. |
 | `mavcaldav` | CalDAV calendar sync. |
+| `mavmaild` | Mail reader (IMAP, read-only). Holds the IMAP password; core never sees it. |
 
 Daemons are wired socket-to-socket, not linked. `internal/ipc` is the client/server wire
 protocol; the config in `deploy/mavend.json` (with `${VAR}` env expansion from gitignored