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>

src/scribe/models/project.py

@@ -1,5 +1,6 @@
 import enum
 from sqlalchemy import BigInteger, ForeignKey, Integer, Text
+from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import Mapped, mapped_column
 from scribe.models import Base
 from scribe.models.base import SoftDeleteMixin, TimestampMixin, iso
@@ -36,6 +37,14 @@ class Project(Base, TimestampMixin, SoftDeleteMixin):
         BigInteger, ForeignKey("forge_connections.id", ondelete="SET NULL"),
         nullable=True,
     )
+    # The inception record (milestone 297): what this project was decided to
+    # inherit, when, and through which door — {decided_at, decided_by, via,
+    # choices: {exclude_always_on_rulebooks, subscribe_rulebooks,
+    # design_system_id, seed_systems}}. NULL means nobody has decided yet,
+    # and enter_project asks; the effects themselves live in the subscription
+    # / exclusion tables, design_system_id and the project's Systems — this is
+    # the WHY, kept so later surfaces can say it. See services/inception.py.
+    inception: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
 
     def to_dict(self) -> dict:
         return {
@@ -48,6 +57,7 @@ class Project(Base, TimestampMixin, SoftDeleteMixin):
             "color": self.color,
             "design_system_id": self.design_system_id,
             "forge_connection_id": self.forge_connection_id,
+            "inception": self.inception,
             "created_at": iso(self.created_at),
             "updated_at": iso(self.updated_at),
         }