feat(inception): projects.inception record + project_rulebook_exclusions — migration 0085 with legacy backfill; backup v10 (#2879, milestone 297 step 1)

A project's inheritance becomes a decision, not a default (milestone 297).
- projects.inception (JSONB, NULL = undecided): {decided_at, decided_by, via
  mcp|ui|legacy, choices {exclude_always_on_rulebooks, subscribe_rulebooks,
  design_system_id, seed_systems}}; on to_dict.
- project_rulebook_exclusions: a project's opt-out of a whole always-on
  rulebook — the sibling of the rule/topic suppressions, CASCADE both ways.
- services/inception.py (first cut): the vocabulary, validate_inception
  (pure, all-or-nothing), normalize_choices, is_decided. Effects come in
  step 3.
- Migration 0085 backfills every existing project via="legacy" with its
  current standing (no exclusions, its subscriptions, its design_system_id,
  no seed) so the ask fires only for projects created after this ships.
- Backup v10: rulebook_exclusions section; project rows carry inception and
  design_system_id, restored in a post-pass once rulebooks/design systems are
  mapped (design_system_id was not restored before — fixed in passing).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_services_backup.py

@@ -18,7 +18,7 @@ def test_backup_version_is_v8():
     point of the test — a payload section added without moving the version
     produces backups that are structurally different and indistinguishable
     by inspection."""
-    assert backup.BACKUP_VERSION == 9
+    assert backup.BACKUP_VERSION == 10
 
 
 def test_not_included_lists_the_known_gaps():
@@ -116,7 +116,7 @@ async def test_export_full_backup_contains_every_declared_section():
                 "systems", "record_systems", "design_systems",
                 "design_tokens", "note_usage_events", "repo_bindings",
                 "note_supersessions", "code_shapes", "code_shape_events",
-                "code_shape_uses"):
+                "code_shape_uses", "rulebook_exclusions"):
         assert key in out, f"missing export section: {key}"
         assert out[key] == []