refactor(comments): hoist NOTE_TITLE to module level

The string "Comment text" is the dedup contract between
cli/comments.attach_callback, bib/sync._sync_notes, and the
upcoming migration script. Pull it out of the function-local scope
into a module constant so consumers can import it instead of
re-hardcoding the literal.

Surfaced by code review on commit 2f6e2d4.

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

src/cli/comments.py

@@ -21,6 +21,11 @@ log = logging.getLogger(__name__)
 
 _DEFAULT_ROOT = Path(".state/comments")
 
+# Title used for the single combined-md note attached per comment item.
+# The migration script and bib/sync dedup also rely on this exact string —
+# treat as a stable contract.
+NOTE_TITLE = "Comment text"
+
 
 def _build_bib_helpers(use_bib: bool, attach: bool):
     """Return ``(body_lookup, attach_callback)``.
@@ -65,8 +70,7 @@ def _build_bib_helpers(use_bib: bool, attach: bool):
     from rex.comments.render import render_combined_md
 
     store = connect()
-    # Items that already have a "Comment text" note — keeps re-runs idempotent.
+    # Items that already have a NOTE_TITLE note — keeps re-runs idempotent.
-    NOTE_TITLE = "Comment text"
     items_with_note: set[str] = {
         row[0]
         for row in store._con().execute(