feat(mnemosyne): implement discover() — serendipitous entry exploration (#1271)

- Added discover() method to archive.py (probabilistic, vitality-weighted)
- Added cmd_discover CLI handler with subparser
- Supports: -n COUNT, -t TOPIC, --vibrant flag
- prefer_fading=True surfaces neglected entries

nexus/mnemosyne/archive.py

@@ -1340,6 +1340,74 @@ class MnemosyneArchive:
         results.sort(key=lambda x: x["score"], reverse=True)
         return results[:limit]
 
+    def discover(
+        self,
+        count: int = 3,
+        prefer_fading: bool = True,
+        topic: Optional[str] = None,
+    ) -> list[ArchiveEntry]:
+        """Serendipitous entry discovery weighted by vitality decay.
+
+        Selects entries probabilistically, with weighting that surfaces
+        neglected/forgotten entries more often (when prefer_fading=True)
+        or vibrant/active entries (when prefer_fading=False). Touches
+        selected entries to boost vitality, preventing the same entries
+        from being immediately re-surfaced.
+
+        Args:
+            count: Number of entries to discover (default 3).
+            prefer_fading: If True (default), weight toward fading entries.
+                If False, weight toward vibrant entries.
+            topic: If set, restrict to entries with this topic (case-insensitive).
+
+        Returns:
+            List of ArchiveEntry, up to count entries.
+        """
+        import random
+
+        candidates = list(self._entries.values())
+
+        if not candidates:
+            return []
+
+        if topic:
+            topic_lower = topic.lower()
+            candidates = [e for e in candidates if topic_lower in [t.lower() for t in e.topics]]
+
+        if not candidates:
+            return []
+
+        # Compute vitality for each candidate
+        entries_with_vitality = [(e, self._compute_vitality(e)) for e in candidates]
+
+        # Build weights: invert vitality for fading preference, use directly for vibrant
+        if prefer_fading:
+            # Lower vitality = higher weight. Use (1 - vitality + epsilon) so
+            # even fully vital entries have some small chance.
+            weights = [1.0 - v + 0.01 for _, v in entries_with_vitality]
+        else:
+            # Higher vitality = higher weight. Use (vitality + epsilon).
+            weights = [v + 0.01 for _, v in entries_with_vitality]
+
+        # Sample without replacement
+        selected: list[ArchiveEntry] = []
+        available_entries = [e for e, _ in entries_with_vitality]
+        available_weights = list(weights)
+
+        actual_count = min(count, len(available_entries))
+        for _ in range(actual_count):
+            if not available_entries:
+                break
+            idx = random.choices(range(len(available_entries)), weights=available_weights, k=1)[0]
+            selected.append(available_entries.pop(idx))
+            available_weights.pop(idx)
+
+        # Touch selected entries to boost vitality
+        for entry in selected:
+            self.touch(entry.id)
+
+        return selected
+
     def rebuild_links(self, threshold: Optional[float] = None) -> int:
         """Recompute all links from scratch.
 

nexus/mnemosyne/cli.py

@@ -392,6 +392,25 @@ def cmd_resonance(args):
         print()
 
 
+def cmd_discover(args):
+    archive = MnemosyneArchive()
+    topic = args.topic if args.topic else None
+    results = archive.discover(
+        count=args.count,
+        prefer_fading=not args.vibrant,
+        topic=topic,
+    )
+    if not results:
+        print("No entries to discover.")
+        return
+    for entry in results:
+        v = archive.get_vitality(entry.id)
+        print(f"[{entry.id[:8]}] {entry.title}")
+        print(f"  Topics: {', '.join(entry.topics) if entry.topics else '(none)'}")
+        print(f"  Vitality: {v['vitality']:.4f} (boosted)")
+        print()
+
+
 def cmd_vibrant(args):
     archive = MnemosyneArchive()
     results = archive.vibrant(limit=args.limit)
@@ -499,6 +518,11 @@ def main():
     rs.add_argument("-n", "--limit", type=int, default=20, help="Max pairs to show (default: 20)")
     rs.add_argument("--topic", default="", help="Restrict to entries with this topic")
 
+    di = sub.add_parser("discover", help="Serendipitous entry exploration")
+    di.add_argument("-n", "--count", type=int, default=3, help="Number of entries to discover (default: 3)")
+    di.add_argument("-t", "--topic", default="", help="Filter to entries with this topic")
+    di.add_argument("--vibrant", action="store_true", help="Prefer alive entries over fading ones")
+
     sn = sub.add_parser("snapshot", help="Point-in-time backup and restore")
     sn_sub = sn.add_subparsers(dest="snapshot_cmd")
     sn_create = sn_sub.add_parser("create", help="Create a new snapshot")
@@ -543,6 +567,7 @@ def main():
         "fading": cmd_fading,
         "vibrant": cmd_vibrant,
         "resonance": cmd_resonance,
+        "discover": cmd_discover,
         "snapshot": cmd_snapshot,
     }
     dispatch[args.command](args)