feat(inception): UI — New-project modal step 2, InceptionCard on the project page, Rules tab shows excluded always-on rulebooks; REST exclusion routes (#2883, milestone 297 step 5)

- components/InceptionCard.vue: the one form, two homes — mode="create" in
  the New-project modal's second step (emits the choices; the create carries
  `inception`), mode="decide" on ProjectView for the owner of an undecided
  project (loads that project's defaults, records the decision). Always-on
  rulebooks listed checked (uncheck = exclude), others unchecked (check =
  subscribe), design system select, seed-Systems toggle (disabled once the
  project has Systems). Tokens only; modal canon (#2855); .btn-* canon.
- ProjectView: the card while undecided, one "Inheritance decided <date> via
  … · …" line after; onDecided refreshes the project.
- ProjectRulesTab: "Excluded always-on rulebooks" section with include-back.
- api/inception.ts (types, fetchInceptionDefaults, decideInception);
  api/rulebooks.ts: ApplicableRules.excluded_always_on, exclude/include
  wrappers; REST POST/DELETE /api/projects/<id>/exclusions/rulebooks/<rb>.

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

frontend/src/api/inception.ts

@@ -0,0 +1,42 @@
+/** Project inception (milestone 297): what a project was decided to inherit. */
+import { apiGet, apiPost } from "@/api/client";
+
+export interface InceptionChoices {
+  exclude_always_on_rulebooks: number[];
+  subscribe_rulebooks: number[];
+  design_system_id: number | null;
+  seed_systems: boolean;
+}
+
+export interface InceptionRecord {
+  decided_at: string;
+  decided_by: number | null;
+  via: "mcp" | "ui" | "legacy";
+  choices: InceptionChoices;
+}
+
+export interface InceptionDefaults {
+  always_on_rulebooks: { id: number; title: string }[];
+  other_rulebooks: { id: number; title: string }[];
+  excluded_always_on: { id: number; title: string }[];
+  subscribed_rulebooks: { id: number; title: string }[];
+  design_system_id: number | null;
+  design_systems: { id: number; title: string }[];
+  systems: number;
+}
+
+export interface InceptionDecision {
+  project_id: number;
+  inception: InceptionRecord;
+  effects: { excluded: number[]; subscribed: number[]; design_system_id: number | null; systems_seeded: string[] };
+}
+
+export const emptyChoices = (): InceptionChoices => ({
+  exclude_always_on_rulebooks: [], subscribe_rulebooks: [], design_system_id: null, seed_systems: false,
+});
+
+export const fetchInceptionDefaults = (projectId: number) =>
+  apiGet<InceptionDefaults>(`/api/projects/${projectId}/inception/defaults`);
+
+export const decideInception = (projectId: number, choices: InceptionChoices) =>
+  apiPost<InceptionDecision>(`/api/projects/${projectId}/inception`, { choices });