fix(hooks): pin timestamp in collision test, avoid dead entry construction, reuse blockPreSet

1. Slug-collision test: Pin event.timestamp to a fixed Date in addition
   to Math.random, preventing flaky behavior when wall-clock crosses a
   second boundary between event1 and event2.

2. handler.ts: Skip entry construction when blockPreSet is true (the
   value is discarded — writtenEntry will be null regardless). Avoids
   misleading dead code.

3. handler.ts: Reuse blockPreSet in the inline-write guard instead of
   re-evaluating context.blockSessionSave === true. Single source of
   truth for the pre-set block condition.

src/hooks/bundled/session-memory/handler.test.ts

@@ -659,11 +659,14 @@ describe("session-memory hook", () => {
       content: createMockSessionContent([{ role: "user", content: "first session" }]),
     });
 
-    // Pin Math.random to force deterministic slug — both handler calls
-    // produce the same fallback filename, exercising the slug-collision
-    // restoration path (preExistingContent !== null).
+    // Pin Math.random AND timestamp to force deterministic slug — both
+    // handler calls produce the same fallback filename, exercising the
+    // slug-collision restoration path (preExistingContent !== null).
+    // Without pinning the clock, a wall-clock second boundary between
+    // event1 and event2 would produce different HHMMSS prefixes → no collision.
     const origRandom = Math.random;
     Math.random = () => 0.5;
+    const fixedTimestamp = new Date("2024-01-15T12:34:56.000Z");
 
     try {
       // First handler: creates memory file with deterministic slug.
@@ -671,6 +674,7 @@ describe("session-memory hook", () => {
         cfg: { agents: { defaults: { workspace: tempDir } } } satisfies OpenClawConfig,
         previousSessionEntry: { sessionId: "s1", sessionFile },
       });
+      event1.timestamp = fixedTimestamp;
       await handler(event1);
       await drainPostHookActions(event1);
 
@@ -692,6 +696,7 @@ describe("session-memory hook", () => {
         cfg: { agents: { defaults: { workspace: tempDir } } } satisfies OpenClawConfig,
         previousSessionEntry: { sessionId: "s2", sessionFile: sessionFile2 },
       });
+      event2.timestamp = fixedTimestamp;
       await handler(event2);
 
       // Verify the file was overwritten by second handler.

src/hooks/bundled/session-memory/handler.ts

@@ -354,6 +354,8 @@ const saveSessionToMemory: HookHandler = async (event) => {
 
     // Use custom content from upstream hook if available, otherwise build entry.
     // hasCustomContent (set above) already gates session loading + slug generation.
+    // When blockPreSet is true, skip entry construction entirely — the inline
+    // write won't happen and the value would be discarded.
     let entry: string;
     if (hasCustomContent) {
       // An empty string is a valid redaction signal — hooks may intentionally
@@ -362,7 +364,7 @@ const saveSessionToMemory: HookHandler = async (event) => {
       log.debug("Using custom session content from upstream hook", {
         length: entry.length,
       });
-    } else {
+    } else if (!blockPreSet) {
       const entryParts = [
         `# Session: ${dateStr} ${timeStr} UTC`,
         "",
@@ -377,6 +379,8 @@ const saveSessionToMemory: HookHandler = async (event) => {
       }
 
       entry = entryParts.join("\n");
+    } else {
+      entry = ""; // Block pre-set — writtenEntry will be null regardless.
     }
 
     // Write inline (fail-safe: if postHookActions never drains, the file
@@ -387,7 +391,7 @@ const saveSessionToMemory: HookHandler = async (event) => {
     // retraction can restore it instead of deleting — preventing accidental
     // erasure of prior memory files when LLM slugs collide on the same day.
     let preExistingContent: string | null = null;
-    if (context.blockSessionSave === true) {
+    if (blockPreSet) {
       log.debug("Session save blocked by upstream hook (inline check)");
     } else {
       await fs.mkdir(memoryDir, { recursive: true });