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>

src/scribe/mcp/server.py

@@ -160,6 +160,9 @@ _READ_ONLY_TOOLS = frozenset({
     # retrieval_telemetry's reason, and needed by a read key so that a line
     # naming a moment can be understood by whoever was shown it.
     "list_moments",
+    # The platform catalog and a project's answers (milestone 463). A pure
+    # read; set_project_platforms is the write.
+    "list_platforms",
     # The pass over the corpus and its queue (milestone 458 step 7): which
     # rules are unjudged and which proposals wait. Reads of the caller's own
     # rules, as list_rules is.
@@ -183,6 +186,7 @@ _WRITE_TOOLS = frozenset({
     # projects, Systems, repos
     "create_project", "update_project", "delete_project", "decide_project_inception",
     "create_system", "update_system", "delete_system", "map_system_to_canonical",
+    "set_project_platforms",
     "bind_repo", "unbind_repo",
     # snippets, processes, the shape ledger
     "create_snippet", "update_snippet", "delete_snippet", "verify_snippet",