Give every prompt one shared context block

The "address him as ты" rule had only reached two of the five system
prompts. Instead of pasting it into the other three (five copies drift —
that is how this happened), there is now one block, in internal/persona,
prepended to all five: nudges, action replies, chat, note queries and
general knowledge.

The block says who he is and how to address him (a man, always "ты",
never "вы", never "он" about him; Maven stays feminine), plus the
current local date and time. It is rendered fresh each turn because the
time changes, and it is correct with an empty config — the address and
gender rules are defaults in code. Config only adds optional facts:
owner_name, city, and the existing free-text `persona` string, which is
now the static half of the block.

Russian even in front of the English prompts: the rules are Russian
grammar, so they read best stated in Russian, and there is one copy.

Vikunja #394.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

cmd/mavend/replier_llm.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/kami/maven/internal/llm"
+	"github.com/kami/maven/internal/persona"
 	"github.com/kami/maven/internal/router"
 	"github.com/kami/maven/internal/voice"
 )
@@ -23,10 +24,14 @@ type completer interface {
 type llmReplier struct {
 	c    completer
 	stub *voice.StubReplier
+
+	// block renders the shared context block per turn (who he is, the time).
+	// nil ⇒ the prompt stands alone.
+	block func() string
 }
 
-func newLLMReplier(c completer) *llmReplier {
-	return &llmReplier{c: c, stub: voice.NewStubReplier()}
+func newLLMReplier(c completer, block func() string) *llmReplier {
+	return &llmReplier{c: c, stub: voice.NewStubReplier(), block: block}
 }
 
 const replySystem = `Ты — Maven, домашняя ассистентка (о себе — в женском роде). Владелец — мужчина, говоришь с ним на "ты", в единственном числе; никогда не "вы"/"ваш" и не "он"/"его". Подтверди действие РОВНО ОДНИМ коротким предложением (≤120 символов), тепло и по-русски. Не задавай вопросов, не повторяй слова, не добавляй ничего после точки. Отвечай ТОЛЬКО одним объектом JSON с полями "response" (текст) и "mood" (ровно одно из: neutral, happy, thinking, tired, confused).
@@ -40,7 +45,7 @@ func (r *llmReplier) Reply(d router.Decision) string {
 	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
 	defer cancel()
 	out, err := r.c.Complete(ctx, llm.Req{
-		System:        replySystem,
+		System:        persona.Prepend(r.block, replySystem),
 		User:          replyContext(d),
 		MaxTokens:     512,
 		RepeatPenalty: 1.3,