refactor(dreamer): rename current_document_count → current_explicit_count

Loop 3 follow-up on d4e10e3. After Loop 2's filter landed, the local in
check_and_schedule_dream held an explicit-only count but was still named
current_document_count — asymmetric with execute_dream's current_explicit_count
(line 201) and contradicting the filter on line 269 that produces the value.

Pure rename: three occurrences (definition at 271, subtraction at 274, log
extra key at 282). No test references. Naming-as-invariant alignment with
d76627a (query filters), d4e10e3 (parameter docstring), and Loop 1's local
rename in execute_dream.

The persisted JSONB key last_dream_document_count is the one remaining
drift-layer; filed as plastic-claudebook backlog item for a separate PR
with an intentional migration path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/dreamer/dream_scheduler.py

@@ -268,10 +268,10 @@ async def check_and_schedule_dream(
         models.Document.observed == collection.observed,
         models.Document.level == "explicit",
     )
-    current_document_count = int(await db.scalar(count_stmt) or 0)
+    current_explicit_count = int(await db.scalar(count_stmt) or 0)
 
     # Calculate documents added since last dream
-    documents_since_last_dream = current_document_count - last_dream_document_count
+    documents_since_last_dream = current_explicit_count - last_dream_document_count
 
     logger.debug(
         "Dream check",
@@ -279,7 +279,7 @@ async def check_and_schedule_dream(
             "workspace_name": collection.workspace_name,
             "observer": collection.observer,
             "observed": collection.observed,
-            "current_document_count": current_document_count,
+            "current_explicit_count": current_explicit_count,
             "last_dream_document_count": last_dream_document_count,
             "documents_since_last_dream": documents_since_last_dream,
             "document_threshold": settings.DREAM.DOCUMENT_THRESHOLD,