feat(plugin): resolve session project from git remote, not a pinned project_id

The SessionStart hook asked for a project_id via plugin userConfig, which pins
one install to a single project — wrong for an operator working across many
repos/projects. Resolve the active project server-side from the working repo's
git remote instead (a stable identifier, not a dir-name guess).

- repo_bindings table (migration 0064) + RepoBinding model: (user, repo_key) ->
  project, FKs CASCADE.
- services/repo_bindings: normalize_repo_key collapses ssh/https/scp/creds/port/
  .git to host/owner/repo; resolve/set/list/delete.
- GET /api/plugin/context takes ?repo=<remote>; unbound repo -> a "bind this
  repo" hint with a ready bind_repo() call. project_id kept as manual override.
- MCP tools: bind_repo / list_repo_bindings / unbind_repo.
- Hook sends ?repo=$(git remote get-url origin) URL-encoded; all project_id
  handling removed. plugin.json drops the project_id userConfig (0.1.2 -> 0.1.3).
- Tests: normalize equivalence classes + unbound-hint rendering.

Refs task 755 (Scribe-as-plugin push channel).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/scribe/models/__init__.py

@@ -40,3 +40,4 @@ from scribe.models.user_profile import UserProfile  # noqa: E402, F401
 from scribe.models.rulebook import (  # noqa: E402, F401
     Rulebook, RulebookTopic, Rule, project_rulebook_subscriptions,
 )
+from scribe.models.repo_binding import RepoBinding  # noqa: E402, F401

src/scribe/models/repo_binding.py

@@ -0,0 +1,40 @@
+from sqlalchemy import ForeignKey, Integer, Text, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column
+
+from scribe.models import Base
+from scribe.models.base import TimestampMixin
+
+
+class RepoBinding(Base, TimestampMixin):
+    """Maps a git repository to the Scribe project it represents.
+
+    The SessionStart hook sends the working repo's normalized remote
+    (`repo_key`, e.g. ``git.fabledsword.com/bvandeusen/fabledscribe``) and the
+    server resolves it to a project — so the operator can work across many
+    repos/projects without pinning a project id in plugin config. The key is a
+    *stable identifier* (the git remote), not a dir-name heuristic.
+    """
+
+    __tablename__ = "repo_bindings"
+    __table_args__ = (
+        UniqueConstraint("user_id", "repo_key", name="uq_repo_bindings_user_repo"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False
+    )
+    project_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("projects.id", ondelete="CASCADE"), nullable=False
+    )
+    repo_key: Mapped[str] = mapped_column(Text, nullable=False)
+
+    def to_dict(self) -> dict:
+        return {
+            "id": self.id,
+            "user_id": self.user_id,
+            "project_id": self.project_id,
+            "repo_key": self.repo_key,
+            "created_at": self.created_at.isoformat(),
+            "updated_at": self.updated_at.isoformat(),
+        }