FN-5762: add mission hierarchy delete tools with linked-task guards

Add mission delete tooling across core, dashboard routes, and CLI extension surfaces.

- Add `fn_feature_delete`, `fn_slice_delete`, and `fn_milestone_delete` tools to the CLI extension with `force` override support.
- Enforce live-task linkage guards for feature/slice/milestone deletes in mission store logic and expose conflict behavior through mission routes.
- Extend coverage and docs for delete behavior, allowlist/gating metadata, and published Fusion skill/tool references.

Files changed:
 .changeset/fn-5762-feature-delete.md               |   7 +
 docs/missions.md                                   |  15 +++
 packages/cli/skill/fusion/SKILL.md                 |   2 +-
 .../cli/skill/fusion/references/extension-tools.md |  27 ++++
 .../skill/fusion/references/fusion-capabilities.md |   3 +
 packages/cli/src/__tests__/extension.test.ts       |  34 +++++
 packages/cli/src/extension.ts                      | 106 +++++++++++++++
 packages/core/src/__tests__/mission-store.test.ts  | 146 ++++++++++++++++++++-
 packages/core/src/mission-store.ts                 |  98 ++++++++++++--
 .../dashboard/src/__tests__/mission-e2e.test.ts    |  62 +++++++--
 packages/dashboard/src/mission-routes.ts           |  42 +++++-
 .../workflow-step-readonly-allowlist.test.ts       |   3 +
 packages/engine/src/gating-classifications.ts      |   3 +
 13 files changed, 514 insertions(+), 34 deletions(-)

Fusion-Task-Id: FN-5762
Fusion-Task-Lineage: 9c6eceff-dcb4-4a2b-90c6-59cb0228ca6d

packages/engine/src/gating-classifications.ts

@@ -33,6 +33,9 @@ const PERMANENT_TASK_AGENT_ONLY_TOOLS = [
   "fn_milestone_add",
   "fn_slice_add",
   "fn_feature_add",
+  "fn_feature_delete",
+  "fn_slice_delete",
+  "fn_milestone_delete",
   "fn_slice_activate",
   "fn_feature_link_task",
   "fn_feature_update",