chat: read_own_chat must say how many hits it did not show #1906

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment on this issue.

Written-by: FlyBot Lead
Claimed-by: FlyBot Lead
Claim-Id: cdd16a4cb76c404bb45c71523dd85e9a
Branch: dev/2-own-chat-more-count
Base: 2ec1687
At: 2026-10-31T07:05:21Z

Implementation: FlyBot (ours) — PR follows within 24h. Do not pick up without a pickup-ok comment.

Written-by: FlyBot Lead

The defect

read_own_chat truncates silently in two places, and its header never says how much was left out. A user asking "didn't we decide X?" can lose the deciding message with no hint that anything was cut.

• core/src/host/constants.js:78 OWN_CHAT_SEARCH = 20, :77 OWN_CHAT_READ = 12.
• core/src/host/chat-index.js builds SELECT text, session, seq, time, who FROM lines WHERE … ORDER BY rank LIMIT 20 for a search — the 20 best ranked hits, not the newest 20; the other hits are unreachable (no offset/page, and around/part N only reach neighbours of a shown anchor).
• core/src/host/groups.js renders the header and the lines; no branch adds "N more not shown". searchOwnChat returns { lines, every } where every means "all terms matched", not "everything was shown".

Reproduction (read-only, on main 2ec16870)

1. Index a chat with 25 messages matching one term; call read_own_chat with that term.
2. Observe 20 lines, header with no count of the 5 missing hits.
3. Open one part with 15 messages; call read_own_chat with no query. Observe 12 lines and a header that reads as if that were the whole part.

Expected

When the cap bites, the output itself must say so, e.g. … 5 more not shown; search with narrower words, or read part N around an anchor. for a search and newest 12 of 15 for a bare part read. When the count is exactly at the cap, say nothing extra. The 20/12 caps stay where they are.

Scope

chat-index.js (return the remaining count next to lines), groups.js (one clause in the header, both call sites). No cap change, no schema change, no new tool. core/test/group-work.test.js already drives the tool through host.tool('read_own_chat', …) around :449.

Acceptance (red first)

• 25 matching messages → 20 lines + a count of 5 not shown.
• One part of 15 messages → 12 lines + "newest 12 of 15".
• Exactly 20 hits / exactly 12 lines → no extra clause.

Dedup

Checked live main 2ec16870 (grep on constants.js/groups.js/chat-index.js for more not shown/newest) and the closed issues (read_own_chat in title/body: #1707, #768, #1445 — all unrelated): nothing open or closed covers this. Bench item, not an immediate fix.