feat(family): platforms declared at inception and detected from bound repos (milestone 463 step 2, #4988)

A project's platforms decide which family ideas reach it. This step makes membership answerable from every door:

- services/platforms.py: the global catalog (writes are admin-only and duplicate-gated by slug); pure marker detection; and membership reads and writes. Detection only ADDS, and only where nobody has answered. It never overrides a declared or rejected row and never removes one.
- coverage: the archive scan now carries every path, and the refresh runs detection fail-open.
- inception: a platforms choice (slugs, or null for unanswered). The list is the whole answer: members left out of it become rejected.
- MCP: list_platforms and set_project_platforms; enter_project and get_project carry the project's platforms.
- REST: /api/platforms (admin writes) and /api/projects/<id>/platforms.
- UI: a platforms checklist on the inception card, a Family tab on ProjectView, and a Platforms admin tab in Settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_mcp_tool_projects.py

@@ -25,6 +25,18 @@ def _no_systems():
         yield
 
 
+@pytest.fixture(autouse=True)
+def _no_platforms():
+    """enter_project and get_project carry the project's platforms (milestone
+    463). No database in this lane, so the lookup is stubbed to the common
+    case — a project with none yet. The populated shape is asserted in its
+    own test.
+    """
+    with patch("scribe.mcp.tools.projects.platforms_svc.project_platforms",
+               AsyncMock(return_value=[])):
+        yield
+
+
 @pytest.fixture(autouse=True)
 def _no_coverage():
     """enter_project also reads the pattern-coverage cache (#2692) — same