feat(FN-4108): persist scope override fields (+4 more)

Commits merged:
- test(FN-4073): finalize invariant regression coverage
- feat(FN-4073): complete Step 4 — document merge invariant
- feat(FN-4073): complete Step 3 — enforce squash file-scope invariant
- feat(FN-4073): complete Step 2 — add file-scope invariant helper
- feat(FN-4073): complete Step 1 — persist scope override fields

Files changed:
.changeset/FN-4073-file-scope-invariant.md         |   9 +
 AGENTS.md                                          |   6 +
 packages/core/src/__tests__/store-update.test.ts   |  18 ++
 packages/core/src/db.ts                            |   2 +
 packages/core/src/store.ts                         |  34 +-
 packages/core/src/types.ts                         |   8 +
 .../__tests__/merger-file-scope-invariant.test.ts  | 357 +++++++++++++++++++++
 .../engine/src/__tests__/merger-test-helpers.ts    |   5 +
 packages/engine/src/merger.ts                      | 155 ++++++++-
 9 files changed, 582 insertions(+), 12 deletions(-)

Fusion-Task-Id: FN-4108

packages/core/src/db.ts

@@ -250,6 +250,8 @@ CREATE TABLE IF NOT EXISTS tasks (
   modifiedFiles TEXT DEFAULT '[]',
   missionId TEXT,
   sliceId TEXT,
+  scopeOverride INTEGER,
+  scopeOverrideReason TEXT,
   assignedAgentId TEXT,
   pausedByAgentId TEXT,
   assigneeUserId TEXT,