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/services/inception.py

@@ -0,0 +1,84 @@
+"""Project inception — what a project was decided to inherit (milestone 297).
+
+A project's inheritance is a decision, not a default. The record lives on
+``projects.inception``::
+
+    {
+      "decided_at": "<iso>", "decided_by": <user id> | null,
+      "via": "mcp" | "ui" | "legacy",
+      "choices": {
+        "exclude_always_on_rulebooks": [rulebook ids],
+        "subscribe_rulebooks": [rulebook ids],
+        "design_system_id": <id> | null,
+        "seed_systems": bool
+      }
+    }
+
+NULL = undecided → enter_project asks. ``legacy`` is the migration's stamp on
+projects that existed before the step did (inherit-all / no design system /
+no seed), so the ask fires only for projects created after this shipped.
+
+Step 1 (this module's first cut) holds the shape and its validator; the
+effects (decide / current_defaults) arrive in step 3 and compose the
+existing services — subscriptions, exclusions, set_project_design_system,
+the Systems starter mint — and write the record LAST.
+"""
+from __future__ import annotations
+
+INCEPTION_VIAS = ("mcp", "ui", "legacy")
+CHOICE_KEYS = ("exclude_always_on_rulebooks", "subscribe_rulebooks", "design_system_id", "seed_systems")
+
+
+def _is_id_list(value) -> bool:
+    return isinstance(value, list) and all(
+        isinstance(v, int) and not isinstance(v, bool) and v > 0 for v in value
+    )
+
+
+def validate_inception(choices) -> str | None:
+    """The structural error an inception ``choices`` object would earn, or
+    None. Pure and checked BEFORE any effect is applied: a decision either
+    applies whole or errors whole (the StrictArgs lesson, #2709).
+
+    Accepts the four keys, each optional: two id lists (positive ints, no
+    duplicates between exclude and subscribe), ``design_system_id`` an int
+    or None, ``seed_systems`` a bool. Unknown keys are an error — a typo
+    must not become a silently ignored choice."""
+    if not isinstance(choices, dict):
+        return "choices must be an object"
+    unknown = sorted(set(choices) - set(CHOICE_KEYS))
+    if unknown:
+        return f"unknown inception choice(s): {', '.join(unknown)} (one of: {', '.join(CHOICE_KEYS)})"
+    excl = choices.get("exclude_always_on_rulebooks") or []
+    subs = choices.get("subscribe_rulebooks") or []
+    if not _is_id_list(excl):
+        return "exclude_always_on_rulebooks must be a list of rulebook ids"
+    if not _is_id_list(subs):
+        return "subscribe_rulebooks must be a list of rulebook ids"
+    both = sorted(set(excl) & set(subs))
+    if both:
+        return f"rulebook(s) {both} cannot be both excluded and subscribed"
+    ds = choices.get("design_system_id")
+    if ds is not None and (isinstance(ds, bool) or not isinstance(ds, int) or ds <= 0):
+        return "design_system_id must be a positive id or null"
+    seed = choices.get("seed_systems", False)
+    if not isinstance(seed, bool):
+        return "seed_systems must be true or false"
+    return None
+
+
+def normalize_choices(choices: dict | None) -> dict:
+    """The four keys, always present, in canonical form — what gets stored
+    and what the UI/agent reads back. Call after validate_inception."""
+    choices = choices or {}
+    return {
+        "exclude_always_on_rulebooks": sorted(set(choices.get("exclude_always_on_rulebooks") or [])),
+        "subscribe_rulebooks": sorted(set(choices.get("subscribe_rulebooks") or [])),
+        "design_system_id": choices.get("design_system_id"),
+        "seed_systems": bool(choices.get("seed_systems", False)),
+    }
+
+
+def is_decided(project) -> bool:
+    """A project is decided once its inception record exists (any via)."""
+    return bool(getattr(project, "inception", None))