Turn a mail into task candidates, and into nothing else (#246)

The extraction half. internal/email.Extractor asks the resident Qwen3-1.7B,
under a GBNF grammar, what one message requires of him, and returns at most
three short candidates with an optional date.

Everything it can produce is a row in `tasks` with status "candidate",
written through the intake seam #130 built for exactly this (Source
"email:<mailbox>", Evidence = the subject line). No reminder, no fact, no
note, no calendar event. That bound is the design: a reminder FIRES, so a
1.7B misreading "встреча была в четверг" as a future appointment would wake
him up about it, whereas a wrong candidate is a line he dismisses in one
click. A due date the model read out of the mail is stored on the candidate,
where no scheduler reads it — the review page sorts by it. Relative wording
("до пятницы") is deliberately left in the text rather than resolved to a
date the model would get wrong.

The prompt is written against the two things a small model does here: it
summarises when asked to extract, and it invents an obligation out of a
polite closing line. Hence the demand for a verb phrase, and an explicit
empty array — most mail contains no task, and a model with no way to say
"nothing" says something.

Wiring: core owns extraction because llama-server lives in core's process,
so the reader hands messages over a new ipc.MethodIngestMail. It is a Server
hook (like StepUp/UnlockFn), not a CoreAPI method — not a store operation,
and no CoreAPI implementation should have to carry it. The hook stays nil
without an `email` config block or without a llama-server phraser, so the
method answers ErrUnknownMethod: off unless configured, twice over. There is
no keyword fallback on purpose — "the subject became a task" is a mailbox
rendered as a to-do list, not extraction.

Privacy: junk is refused before the model is called, mail text is never
search input, extraction errors carry byte counts rather than the reply, the
stored evidence is a truncated subject, and the log line names the mailbox
and the UID only.

internal/ipc/client.go

@@ -448,6 +448,17 @@ func (c *Client) SetTaskStatus(ctx context.Context, id int64, status string, ts
 	return c.call(ctx, MethodSetTaskStatus, setTaskStatusReq{ID: id, Status: status, Ts: ts}, nil)
 }
 
+// IngestMail hands one fetched message to core for extraction. ErrUnknownMethod
+// means core has no email block configured — the caller should stop asking, not
+// retry.
+func (c *Client) IngestMail(ctx context.Context, req IngestMailReq) (IngestMailResp, error) {
+	var r IngestMailResp
+	if err := c.call(ctx, MethodIngestMail, req, &r); err != nil {
+		return IngestMailResp{}, err
+	}
+	return r, nil
+}
+
 func (c *Client) DismissProposedRoutine(ctx context.Context, id int64) error {
 	return c.call(ctx, MethodDismissProposedRoutine, dismissProposedRoutineReq{ID: id}, nil)
 }