feat(ledger): a repo binding names the branch its ledger follows — bind_repo(ref=) (#2873, milestone 294)

Project 2 is bound to main, so every consolidation of the 2026-08 audit was
invisible to the ledger until the dev→main merge; the operator works on dev
(rule 1). repo_bindings.ref (migration 0082, nullable) is the branch the
coverage refresh reads; NULL keeps the forge default branch. set_binding takes
ref (name sets, "" clears, None leaves standing); bindings_for_project feeds
the refresh; bind_repo exposes ref ("-" clears). to_dict carries it.

Operator decision on #2873 (2026-08-21): per-binding ref, chosen at bind time,
default the repo default branch.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/models/repo_binding.py

@@ -28,6 +28,10 @@ class RepoBinding(Base, TimestampMixin):
         Integer, ForeignKey("projects.id", ondelete="CASCADE"), nullable=False
     )
     repo_key: Mapped[str] = mapped_column(Text, nullable=False)
+    # The branch the coverage refresh reads for this binding (#2873); NULL =
+    # the forge's default branch. Chosen at bind time so a dev-first project
+    # can have its ledger follow dev instead of waiting for the merge.
+    ref: Mapped[str | None] = mapped_column(Text, nullable=True)
 
     def to_dict(self) -> dict:
         return {
@@ -35,6 +39,7 @@ class RepoBinding(Base, TimestampMixin):
             "user_id": self.user_id,
             "project_id": self.project_id,
             "repo_key": self.repo_key,
+            "ref": self.ref,
             "created_at": iso(self.created_at),
             "updated_at": iso(self.updated_at),
         }