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