FN-5764: expose mission assertion backfill through API and CLI

Expose mission assertion backfill controls and run paths across engine, API, and CLI surfaces.

- add mission assertion backfill gating classification and reliability coverage for assertion-linked validation recovery
- add dashboard mission routes plus legacy API coverage and e2e checks for running assertion backfill
- expose new extension tool support and update Fusion skill docs/capability references
- add a changeset for @runfusion/fusion and refresh mission documentation

Files changed:
 .changeset/fn-5764-mission-backfill-assertions.md  |   9 ++
 docs/missions-completion-contract.md               |   7 +-
 docs/missions.md                                   |   7 +-
 packages/cli/skill/fusion/SKILL.md                 |   2 +-
 .../cli/skill/fusion/references/extension-tools.md |   9 ++
 .../skill/fusion/references/fusion-capabilities.md |   1 +
 packages/cli/src/__tests__/extension.test.ts       |  41 +++++++
 packages/cli/src/extension.ts                      |  43 +++++++
 .../dashboard/app/__tests__/api-missions.test.ts   |  44 +++++++
 packages/dashboard/app/api/legacy.ts               |  34 ++++++
 .../dashboard/src/__tests__/mission-e2e.test.ts    | 127 +++++++++++++++++++++
 packages/dashboard/src/mission-routes.ts           |  29 +++++
 .../mission-validation-trigger-gap.test.ts         |  66 +++++++++++
 .../workflow-step-readonly-allowlist.test.ts       |   1 +
 packages/engine/src/gating-classifications.ts      |   1 +
 15 files changed, 418 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-5764

Fusion-Task-Lineage: b16123c2-7cfe-4b17-a899-4e30044a7a49

packages/engine/src/gating-classifications.ts

@@ -30,6 +30,7 @@ const PERMANENT_TASK_AGENT_ONLY_TOOLS = [
   "fn_mission_create",
   "fn_mission_delete",
   "fn_mission_update",
+  "fn_mission_backfill_assertions",
   "fn_milestone_add",
   "fn_slice_add",
   "fn_feature_add",