FN-5713: add mission update tool and docs

Add mission-update support to Fusion tooling and document it across mission references.

- register new `fn_mission_update` extension tool to patch mission title/description with validation and not-found handling
- add extension tests covering successful mission updates and error cases
- allowlist `fn_mission_update` in workflow-step readonly gating policy tests and classifications
- update Fusion skill and mission docs to describe mission updates and extension tool usage
- add a changeset for `@runfusion/fusion` patch release

Files changed:
 .changeset/fn-5713-mission-update-tool.md          |  5 ++
 docs/missions.md                                   | 13 ++++
 packages/cli/skill/fusion/SKILL.md                 |  2 +-
 packages/cli/skill/fusion/references/extension-tools.md | 10 +++
 packages/cli/skill/fusion/references/fusion-capabilities.md |  1 +
 packages/cli/src/__tests__/extension.test.ts       | 71 ++++++++++++++++++++++
 packages/cli/src/extension.ts                      | 69 +++++++++++++++++++++
 packages/engine/src/__tests__/workflow-step-readonly-allowlist.test.ts |  1 +
 packages/engine/src/gating-classifications.ts      |  1 +
 9 files changed, 172 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-5713
Fusion-Task-Lineage: e491973a-2d5d-4ef8-bf13-39b3669918ee

packages/cli/skill/fusion/SKILL.md

@@ -29,7 +29,7 @@ Mission → Milestone → Slice → Feature → Task
 <!-- BEGIN: tool-categories (auto-generated by scripts/sync-fusion-skill-tools.mjs — do not edit by hand) -->
 - **Task tools** — `fn_task_create`, `fn_task_update`, `fn_task_list`, `fn_task_show`, `fn_task_attach`, `fn_task_pause`, `fn_task_unpause`, `fn_task_retry`, `fn_task_duplicate`, `fn_task_refine`, `fn_task_archive`, `fn_task_unarchive`, `fn_task_delete`, `fn_task_plan`
 - **GitHub tools** — `fn_task_import_github`, `fn_task_import_github_issue`, `fn_task_browse_github_issues`
-- **Mission tools** — `fn_mission_create`, `fn_mission_list`, `fn_mission_show`, `fn_mission_delete`, `fn_milestone_add`, `fn_slice_add`, `fn_feature_add`, `fn_slice_activate`, `fn_feature_link_task`, `fn_feature_update`, `fn_milestone_update`
+- **Mission tools** — `fn_mission_create`, `fn_mission_list`, `fn_mission_show`, `fn_mission_delete`, `fn_mission_update`, `fn_milestone_add`, `fn_slice_add`, `fn_feature_add`, `fn_slice_activate`, `fn_feature_link_task`, `fn_feature_update`, `fn_milestone_update`
 - **Goal tools** — `fn_goal_list`, `fn_goal_create`, `fn_goal_archive`, `fn_goal_show`
 - **Agent tools** — `fn_agent_stop`, `fn_agent_start`, `fn_agent_create`, `fn_agent_delete`, `fn_list_agents`, `fn_delegate_task`, `fn_agent_show`, `fn_agent_org_chart`
 - **Skills tools** — `fn_skills_search`, `fn_skills_install`

packages/cli/skill/fusion/references/extension-tools.md

@@ -204,6 +204,16 @@ Delete a mission and all its milestones, slices, and features. Cannot be undone.
 |-----------|------|----------|-------------|
 | `id` | string | ✓ | Mission ID to delete (e.g., M-001) |
 
+### fn_mission_update
+
+Update an existing mission's title or description. Partial patches leave untouched fields intact.
+
+| Parameter | Type | Required | Description |
+|-----------|------|----------|-------------|
+| `id` | string | ✓ | Mission ID to update (e.g., M-001) |
+| `title` | string | — | Updated mission title |
+| `description` | string | — | Updated mission description |
+
 ### fn_milestone_add
 
 Add a milestone to a mission. Milestones represent phases of work.

packages/cli/skill/fusion/references/fusion-capabilities.md

@@ -49,6 +49,7 @@ All skill/extension tool invocations in this catalog use the public `fn_*` names
 | `fn_goal_show` | Show full details for a single goal by ID. |
 | `fn_mission_show` | Show mission details with full hierarchy: milestones → slices → features. |
 | `fn_mission_delete` | Delete a mission and all its milestones, slices, and features. Cannot be undone. |
+| `fn_mission_update` | Update an existing mission's title or description. Partial patches leave untouched fields intact. |
 | `fn_milestone_add` | Add a milestone to a mission. Milestones represent phases of work. |
 | `fn_slice_add` | Add a slice to a milestone. Slices are work units that can be activated for implementation. |
 | `fn_feature_add` | Add a feature to a slice. Features are deliverables that can be linked to tasks. |