titles: smart truncation — the cut snaps to word boundaries

Owner round (2026-10-06, release polish #3): first messages are always
long, and the auto-title's hard 72-char cut landed mid-word. The cut now
snaps to the last word boundary at or under TITLE_MAX_CHARS (72, exported
and tunable); whitespace collapses first so boundaries are single-space.
The one special case (owner-specified): a single word longer than the cap
— rare but real: identifiers, URLs — splits at the cap. A boundary landing
exactly at the cap is a clean cut. Seam unchanged: titleFromPrompt
(core/contracts/graph.ts), applied once server-side at chat creation.
Pinned in tests/title-from-prompt.test.ts (6).

src/core/contracts/graph.ts

@@ -341,9 +341,34 @@ export interface Position {
 	y: number;
 }
 
+/** The auto-title's character cap (owner, 2026-10-06): long enough to
+ *  stay informative in the sidebar and the top bar, short enough that a
+ *  first message never reads as a paragraph. Tunable — one number, one
+ *  place. */
+export const TITLE_MAX_CHARS = 72;
+
 /** Derive a short chat title from the learner's first message (pure).
+ *  Smart truncation (owner, 2026-10-06): the cut snaps to a word boundary
+ *  AT OR UNDER the cap — a title never ends mid-word. The one exception
+ *  is a single word longer than the cap (rare, but real: identifiers,
+ *  URLs): it cuts at the cap, splitting the word. Whitespace collapses to
+ *  single spaces first, so boundaries are always single-space boundaries.
  *  Used on chat creation; deterministic so tests can pin it. */
 export function titleFromPrompt(prompt: string): string {
 	const oneLine = prompt.replace(/\s+/g, " ").trim();
-	return oneLine.length > 72 ? `${oneLine.slice(0, 72)}…` : oneLine;
+	if (oneLine.length <= TITLE_MAX_CHARS) return oneLine;
+	// Last space among the first TITLE_MAX_CHARS characters; the cut sits
+	// before it so no trailing space rides into the title.
+	let cut = -1;
+	for (let i = TITLE_MAX_CHARS; i > 0; i--) {
+		if (oneLine[i - 1] === " ") {
+			cut = i - 1;
+			break;
+		}
+	}
+	// One long word: no boundary exists under the cap — split it (owner's
+	// special case). This also lands correctly when the first word exactly
+	// fills the cap: cutting at the cap IS the boundary.
+	if (cut < 0) cut = TITLE_MAX_CHARS;
+	return `${oneLine.slice(0, cut)}…`;
 }