fix(agent): derive short session entry ids from the uuidv7 random tail (closes #6242)

The uuidv7 prefix is timestamp-derived and nearly constant between calls, so slicing the first 8 chars degenerated short ids to full-UUID fallbacks. Regression from 80c918c2 which replaced randomUUID with uuidv7.

packages/agent/CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Fixed
 
 - Fixed harness split-turn compaction to serialize summary requests so single-concurrency providers are not asked to run overlapping generations ([#5536](https://github.com/earendil-works/pi/issues/5536)).
+- Fixed harness session storage short entry ids to use the random tail of the generated uuidv7 instead of the timestamp prefix, which was nearly constant between calls ([#6242](https://github.com/earendil-works/pi/issues/6242)).
 
 ## [0.80.3] - 2026-06-30
 

packages/agent/src/harness/session/jsonl-storage.ts

@@ -34,7 +34,9 @@ function buildLabelsById(entries: SessionTreeEntry[]): Map<string, string> {
 
 function generateEntryId(byId: { has(id: string): boolean }): string {
 	for (let i = 0; i < 100; i++) {
-		const id = uuidv7().slice(0, 8);
+		// The uuidv7 prefix is timestamp-derived and nearly constant between calls,
+		// so short ids must come from the random tail.
+		const id = uuidv7().slice(-8);
 		if (!byId.has(id)) return id;
 	}
 	return uuidv7();

packages/agent/src/harness/session/memory-storage.ts

@@ -27,7 +27,9 @@ function buildLabelsById(entries: SessionTreeEntry[]): Map<string, string> {
 
 function generateEntryId(byId: { has(id: string): boolean }): string {
 	for (let i = 0; i < 100; i++) {
-		const id = uuidv7().slice(0, 8);
+		// The uuidv7 prefix is timestamp-derived and nearly constant between calls,
+		// so short ids must come from the random tail.
+		const id = uuidv7().slice(-8);
 		if (!byId.has(id)) return id;
 	}
 	return uuidv7();