feat(mnemosyne): add tag management — add_tags, remove_tags, retag

Closes #1236.

Three new methods on MnemosyneArchive:
- add_tags: add new tags (dedup, case-insensitive)
- remove_tags: remove specific tags
- retag: replace all tags at once

All methods return the entry's final topic list.

nexus/mnemosyne/archive.py

@@ -212,6 +212,79 @@ class MnemosyneArchive:
     def count(self) -> int:
         return len(self._entries)
 
+
+
+    def add_tags(self, entry_id: str, tags: list[str]) -> list[str]:
+        """Add tags to an existing entry. Returns the entry's full topic list after addition.
+
+        Args:
+            entry_id: The entry ID to modify.
+            tags: List of tags to add (duplicates are ignored).
+
+        Raises:
+            KeyError: If entry_id does not exist.
+        """
+        entry = self._entries.get(entry_id)
+        if entry is None:
+            raise KeyError(f"Entry not found: {entry_id}")
+
+        existing_lower = {t.lower() for t in entry.topics}
+        for tag in tags:
+            tag = tag.strip()
+            if tag and tag.lower() not in existing_lower:
+                entry.topics.append(tag)
+                existing_lower.add(tag.lower())
+
+        self._save()
+        return entry.topics
+
+    def remove_tags(self, entry_id: str, tags: list[str]) -> list[str]:
+        """Remove tags from an existing entry. Returns the entry's topic list after removal.
+
+        Args:
+            entry_id: The entry ID to modify.
+            tags: List of tags to remove (case-insensitive).
+
+        Raises:
+            KeyError: If entry_id does not exist.
+        """
+        entry = self._entries.get(entry_id)
+        if entry is None:
+            raise KeyError(f"Entry not found: {entry_id}")
+
+        remove_lower = {t.strip().lower() for t in tags}
+        entry.topics = [t for t in entry.topics if t.lower() not in remove_lower]
+
+        self._save()
+        return entry.topics
+
+    def retag(self, entry_id: str, tags: list[str]) -> list[str]:
+        """Replace all tags on an existing entry. Returns the new topic list.
+
+        Args:
+            entry_id: The entry ID to modify.
+            tags: The new complete tag list (replaces all existing tags).
+
+        Raises:
+            KeyError: If entry_id does not exist.
+        """
+        entry = self._entries.get(entry_id)
+        if entry is None:
+            raise KeyError(f"Entry not found: {entry_id}")
+
+        # Deduplicate, preserve order
+        seen: set[str] = set()
+        deduped: list[str] = []
+        for tag in tags:
+            tag = tag.strip()
+            if tag and tag.lower() not in seen:
+                deduped.append(tag)
+                seen.add(tag.lower())
+
+        entry.topics = deduped
+        self._save()
+        return entry.topics
+
     def graph_data(
         self,
         topic_filter: Optional[str] = None,