extract: document basename-uniqueness invariant + skip _-prefixed archive files

Two nits from Ganymede review of ed4af4d:

1. Archive-basename filter depends on basename-uniqueness across queue+archive.
   Current naming (date-prefix + topic-slug) makes collisions rare, but if
   short generic names like "notes.md" enter the queue, the filter silently
   false-positives. Comment block names the assumption.

2. Archive walk now skips _-prefixed files, matching the standing convention
   everywhere else (search.py STRUCTURAL_FILES, reweave wiki-link skip, Layer
   0 entity exclusion). Defensive — no _*.md exists under inbox/archive/
   today, but consistent with codebase convention if a future operator drops
   _README.md to document the directory.

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

lib/extract.py

@@ -930,11 +930,18 @@ async def extract_cycle(conn, max_workers=None) -> tuple[int, int]:
     # copy is the source of truth — if a file with this basename is in archive,
     # the source is processed regardless of queue state. Single archive scan
     # per cycle, cheap (~1k files).
+    #
+    # Assumes basename uniqueness across queue+archive — current naming
+    # convention (date-prefix + topic-slug) makes collisions vanishingly
+    # rare. If short generic names like "notes.md" enter the queue, this
+    # filter silently false-positives.
     if unprocessed:
         archive_dir = main / "inbox" / "archive"
         archived_basenames: set[str] = set()
         if archive_dir.exists():
             for af in archive_dir.rglob("*.md"):
+                if af.name.startswith("_"):
+                    continue
                 archived_basenames.add(af.name)
         if archived_basenames:
             before = len(unprocessed)