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>

alembic/versions/0085_project_inception.py

@@ -0,0 +1,74 @@
+"""Project inception: the decision record + always-on rulebook exclusions (milestone 297)
+
+Revision ID: 0085
+Revises: 0084
+Create Date: 2026-08-22
+
+`projects.inception` is the WHY a project inherits what it does — NULL until
+someone decides, at which point enter_project stops asking. The new
+association `project_rulebook_exclusions` is the opt-out of a whole always-on
+rulebook for one project (the sibling of the rule/topic suppressions).
+
+Backfill: every project that exists when this runs is stamped
+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 the step shipped, and nothing a running install relies on changes.
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision = "0085"
+down_revision = "0084"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "projects",
+        sa.Column("inception", postgresql.JSONB(), nullable=True),
+    )
+    op.create_table(
+        "project_rulebook_exclusions",
+        sa.Column(
+            "project_id", sa.BigInteger(),
+            sa.ForeignKey("projects.id", ondelete="CASCADE"),
+            primary_key=True, nullable=False,
+        ),
+        sa.Column(
+            "rulebook_id", sa.BigInteger(),
+            sa.ForeignKey("rulebooks.id", ondelete="CASCADE"),
+            primary_key=True, nullable=False,
+        ),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True),
+            server_default=sa.text("now()"), nullable=False,
+        ),
+    )
+    # Legacy stamp: what each existing project inherits today, recorded as a
+    # decision so the inception ask does not fire on a project that has been
+    # running for months.
+    op.execute(sa.text("""
+        UPDATE projects p SET inception = jsonb_build_object(
+            'via', 'legacy',
+            'decided_at', to_jsonb(now()),
+            'decided_by', NULL,
+            'choices', jsonb_build_object(
+                'exclude_always_on_rulebooks', '[]'::jsonb,
+                'subscribe_rulebooks', COALESCE(
+                    (SELECT jsonb_agg(s.rulebook_id ORDER BY s.rulebook_id)
+                       FROM project_rulebook_subscriptions s
+                      WHERE s.project_id = p.id),
+                    '[]'::jsonb),
+                'design_system_id', to_jsonb(p.design_system_id),
+                'seed_systems', false
+            )
+        )
+        WHERE p.inception IS NULL
+    """))
+
+
+def downgrade() -> None:
+    op.drop_table("project_rulebook_exclusions")
+    op.drop_column("projects", "inception")