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/mcp/tools/repos.py

@@ -13,28 +13,43 @@ from scribe.services import projects as projects_svc
 from scribe.services import repo_bindings as repo_bindings_svc
 
 
-async def bind_repo(repo_url: str, project_id: int) -> dict:
+async def bind_repo(repo_url: str, project_id: int, ref: str = "") -> dict:
     """Bind a git repository to a Scribe project for session-start context.
 
     After this, any session started in that repo auto-loads the project's
     context (the SessionStart hook sends the repo's remote; the server resolves
     it here). Idempotent — re-binding the same repo updates the target project.
 
+    The binding is also what the shape ledger reads (refresh_pattern_coverage):
+    `ref` names the branch it follows. Default (""): the repo's default branch
+    — which means the ledger only sees work after a merge. A dev-first project
+    (rule 1: dev is home) should bind with ref="dev" so classification follows
+    the push, not the merge. Re-binding with ref="" keeps the standing ref;
+    pass ref="-" to clear it back to the default branch.
+
     Args:
         repo_url: the repo's git remote (e.g. the output of
             `git remote get-url origin` — ssh or https form, both work).
         project_id: the Scribe project this repo represents.
+        ref: branch the ledger follows ("" = leave as is / default branch on
+            a new binding; "-" = clear to the default branch).
     """
     uid = current_user_id()
     project = await projects_svc.get_project(uid, project_id)
     if project is None:
         raise ValueError(f"project {project_id} not found")
-    binding = await repo_bindings_svc.set_binding(uid, repo_url, project_id)
+    ref_arg = None if not ref else ("" if ref.strip() == "-" else ref)
+    binding = await repo_bindings_svc.set_binding(uid, repo_url, project_id, ref_arg)
+    follows = binding.ref or "the default branch"
     return {
         "repo_key": binding.repo_key,
         "project_id": binding.project_id,
         "project_title": project.title,
-        "message": f"Bound `{binding.repo_key}` -> {project.title} (id {project.id}).",
+        "ref": binding.ref,
+        "message": (
+            f"Bound `{binding.repo_key}` -> {project.title} (id {project.id}); "
+            f"the ledger follows {follows}."
+        ),
     }