FN-5896: persist mission-goal linkage in schema

Persist mission↔goal relationships through the core mission schema and store APIs.

- add the mission_goals join table, schema migration coverage, and schema version bump to 101
- add MissionStore goal link/unlink/list helpers plus a MissionGoalLink type and emitted linkage events
- add regression tests for persistence, idempotency, ordering, and cascade behavior, and document the new linkage model with a published changeset

Files changed:
 .changeset/fn-5896-mission-goal-linkage.md         |   5 +
 docs/missions.md                                   |  19 +++
 docs/storage.md                                    |   1 +
 packages/core/src/__tests__/db-migrate.test.ts     |  49 ++++++-
 packages/core/src/__tests__/db.test.ts             |  34 ++---
 packages/core/src/__tests__/goals-schema.test.ts   |   4 +-
 packages/core/src/__tests__/insight-store.test.ts  |  10 +-
 .../src/__tests__/merge-request-record.test.ts     |   2 +-
 .../core/src/__tests__/mission-goals-link.test.ts  | 142 +++++++++++++++++++++
 packages/core/src/__tests__/mission-store.test.ts  |   4 +-
 packages/core/src/__tests__/run-audit.test.ts      |   2 +-
 packages/core/src/__tests__/secrets-schema.test.ts |   6 +-
 .../core/src/__tests__/store-merge-queue.test.ts   |   2 +-
 packages/core/src/__tests__/task-documents.test.ts |   2 +-
 packages/core/src/db.ts                            |  31 ++++-
 packages/core/src/mission-store.ts                 | 111 ++++++++++++++++
 packages/core/src/mission-types.ts                 |   6 +
 .../src/store/__tests__/roadmap-store.test.ts      |   2 +-
 18 files changed, 391 insertions(+), 41 deletions(-)

Fusion-Task-Id: FN-5896

Fusion-Task-Lineage: a4c7c495-b20a-454b-9605-782671cdd8c8

plugins/fusion-plugin-roadmap/src/store/__tests__/roadmap-store.test.ts

@@ -744,7 +744,7 @@ describe("RoadmapStore", () => {
 
   describe("schema version", () => {
     it("schema version is 100 after init", () => {
-      expect(db.getSchemaVersion()).toBe(100);
+      expect(db.getSchemaVersion()).toBe(101);
     });
   });