mavend: answer "а завтра?" from the previous turn, not from the model

An elliptical follow-up carries no intent of its own. followUpMerge cannot
help — it inherits slots once the intent is known, and here the intent is
the missing part. So "а завтра?" went to the router, which on a 1.7B is
close to a coin flip, and the guess cost ~2.7s.

continuationDecision runs before the router and rebuilds the turn from the
previous one: same intent, same key, new day. Deterministic and free.

Three guards, all narrow on purpose. A parseable date is required, which is
what separates an ellipsis from an ordinary short utterance. Four tokens
max. And only query, system and reminder may be inherited: fact and note
would write something he did not say, and act would let a two-word
utterance re-run an allowlisted fn, which is a way to fire a destructive
command nobody typed.

A continuation is still remembered, so "а завтра?" then "а послезавтра?"
chains.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TrVSBKe3RFDF4fGYKWYQnX

cmd/mavend/continuation.go

@@ -0,0 +1,118 @@
+// Elliptical follow-ups — "а завтра?" after "какие напоминания на сегодня".
+//
+// These carry no intent of their own. Two words, one of them a particle, and
+// everything that makes the utterance meaningful lives in the turn before it.
+// Sent to the router they get whatever the model guesses, which on a 1.7B is
+// close to a coin flip, and the guess costs ~2.7s to obtain.
+//
+// followUpMerge (followup.go) cannot help: it inherits SLOTS once the intent is
+// known, and here the intent is the missing part. So this runs before the
+// router and answers from the previous turn directly, which is both correct by
+// construction and free.
+package main
+
+import (
+	"time"
+
+	"github.com/kami/maven/internal/dialogue"
+	"github.com/kami/maven/internal/router"
+)
+
+// continuationMaxTokens — an ellipsis is short by definition. Past four tokens
+// the utterance carries enough of its own content to be routed on its merits,
+// and inheriting an intent for it would be overreach.
+const continuationMaxTokens = 4
+
+// continuationParticles — the words that open a follow-up. A leading particle
+// is one of the two ways in; the other is an utterance that is nothing but a
+// date ("завтра?").
+var continuationParticles = map[string]bool{
+	"а": true, "и": true, "ну": true,
+	"what": true, "and": true, "how": true,
+}
+
+// continuableIntents — which intents an ellipsis may inherit.
+//
+// query and system are questions: asking the same question about a different
+// day is exactly what "а завтра?" means. reminder is an instruction that names
+// a time, so re-aiming it at another day is a coherent second instruction.
+//
+// The rest are excluded on purpose. fact and note would write something he did
+// not say — "поужинал" then "а вчера?" is a question about yesterday, not a
+// claim about it. chat has no slot to re-aim. act is the dangerous one: an
+// allowlisted fn inherited by a two-word utterance is a way to run a
+// destructive command nobody typed, and no follow-up is worth that.
+var continuableIntents = map[dialogue.Intent]bool{
+	dialogue.IntentQuery:    true,
+	dialogue.IntentSystem:   true,
+	dialogue.IntentReminder: true,
+}
+
+// continuationDecision reads an utterance as "the previous question, but for
+// this other day". Returns ok=false whenever anything is uncertain, which
+// hands the turn back to the ordinary router path.
+//
+// The date is what makes this safe. An ellipsis with no parseable day is just
+// a short utterance, and short utterances are the router's job.
+func continuationDecision(prev *dialogue.Session, text string, now time.Time) (router.Decision, bool) {
+	if prev == nil || prev.IsExpired(now) || !continuableIntents[prev.Intent] {
+		return router.Decision{}, false
+	}
+	tokens := quietTokens(text)
+	if len(tokens) == 0 || len(tokens) > continuationMaxTokens {
+		return router.Decision{}, false
+	}
+	day, ok := router.ParseCalendarDate(text, now)
+	if !ok {
+		return router.Decision{}, false
+	}
+	// Either it opens with a particle, or the whole utterance is the date.
+	if !continuationParticles[tokens[0]] && !isBareDate(tokens, day, now) {
+		return router.Decision{}, false
+	}
+
+	dec := router.Decision{
+		Utterance:  text,
+		Intent:     router.Intent(prev.Intent),
+		Confidence: 1.0,
+		Stage:      0,
+		Slots: router.Slots{
+			Key:    prev.Slots.Key,
+			HasKey: prev.Slots.HasKey,
+			Value:  prev.Slots.Value,
+			Text:   prev.Slots.Text,
+			// Fn/Args are deliberately not carried: continuableIntents
+			// excludes act, so there is never one to carry.
+			Time:    day,
+			HasTime: true,
+		},
+	}
+	return dec, true
+}
+
+// isBareDate reports whether the utterance is nothing but its date expression.
+// "завтра" and "на выходных" qualify; "напомни завтра" does not, because the
+// verb is content of its own and belongs to the router.
+//
+// Implemented by re-parsing each token: if every token that is not part of a
+// date expression is a preposition or a question mark's leftovers, the
+// utterance is bare. Cheap enough at four tokens.
+func isBareDate(tokens []string, day time.Time, now time.Time) bool {
+	for _, t := range tokens {
+		if continuationFillers[t] {
+			continue
+		}
+		if d, ok := router.ParseCalendarDate(t, now); ok && d.Equal(day) {
+			continue
+		}
+		return false
+	}
+	return true
+}
+
+// continuationFillers — tokens that carry no content of their own inside a
+// date expression ("на выходных", "в среду").
+var continuationFillers = map[string]bool{
+	"на": true, "в": true, "во": true, "за": true, "про": true,
+	"about": true, "on": true, "for": true,
+}