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

docs/missions.md

@@ -23,6 +23,25 @@ Mission: Improve Reliability
         Task: FN-214
 ```
 
+## Mission ↔ Goal persistence
+
+Missions and goals are stored independently, but Fusion now persists an optional many-to-many linkage in the `mission_goals` join table.
+
+- Columns: `missionId`, `goalId`, `createdAt`
+- Primary key: `(missionId, goalId)`
+- Foreign keys: `missionId → missions.id`, `goalId → goals.id`
+- Delete behavior: both foreign keys use `ON DELETE CASCADE`, so removing either parent deletes only the corresponding join rows
+- Reverse lookups are indexed via `idxMissionGoalsGoalId`
+
+`MissionStore` owns the linkage CRUD surface:
+
+- `linkGoal(missionId, goalId)` — idempotently create a link and return `{ missionId, goalId, createdAt }`
+- `unlinkGoal(missionId, goalId)` — remove a link and report whether anything changed
+- `listGoalIdsForMission(missionId)` — list linked goals in deterministic creation order
+- `listMissionIdsForGoal(goalId)` — list linked missions in deterministic creation order
+
+Existing missions are **not** backfilled with goal links as part of this schema change; that decision is deferred to FN-5898.
+
 ## Creating Missions
 
 ### Mission base branch defaults