feat(FN-4579): complete Step 5 — clarify milestone field semantics

Fusion-Task-Id: FN-4579
Fusion-Task-Lineage: b0f3b436-599f-4b7b-a598-b8789e0e4cb7

docs/missions.md

@@ -122,6 +122,14 @@ When missions are created through the interview planning workflow, Fusion automa
 - **Verification fields**: Milestone and slice verification criteria from the interview are stored in dedicated `verification` fields rather than concatenated into descriptions
 - **Partial plans handled**: Auto-generation is robust to partial plans (missing slices/features or empty criteria) without throwing errors
 
+### Milestone Text Field Semantics
+
+Milestones now carry three complementary free-text fields:
+
+- `description` — narrative scope of the phase
+- `verification` — informal "how to confirm" notes
+- `acceptanceCriteria` — structured acceptance/assertion text (the canonical pass/fail bar), parallel to feature-level `acceptanceCriteria`
+
 ## Slice Activation and Progress
 
 Slices represent staged execution windows.

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

@@ -261,7 +261,7 @@ Update an existing feature's title, description, or acceptance criteria. Partial
 
 ### fn_milestone_update
 
-Update an existing milestone's title, description, or acceptance criteria. Partial patches leave untouched fields intact.
+Update an existing milestone's title, description, or acceptance criteria (the structured pass/fail bar, distinct from verification's free-form how-to-confirm notes). Partial patches leave untouched fields intact.
 
 | Parameter | Type | Required | Description |
 |-----------|------|----------|-------------|

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

@@ -50,7 +50,7 @@ All skill/extension tool invocations in this catalog use the public `fn_*` names
 | `fn_slice_activate` | Activate a pending slice for implementation. Sets status to 'active' and enables task linking for its features. |
 | `fn_feature_link_task` | Link a feature to a fn task for implementation. Updates the feature status to 'triaged' and associates it with the task. |
 | `fn_feature_update` | Update an existing feature's title, description, or acceptance criteria. Partial patches leave untouched fields intact. |
-| `fn_milestone_update` | Update an existing milestone's title, description, or acceptance criteria. Partial patches leave untouched fields intact. |
+| `fn_milestone_update` | Update an existing milestone's title, description, or acceptance criteria (the structured pass/fail bar, distinct from verification's free-form how-to-confirm notes). Partial patches leave untouched fields intact. |
 | `fn_agent_stop` | Stop a running agent — pauses its execution. Transitions the agent from running/active to paused state. |
 | `fn_agent_start` | Start a stopped agent — resumes its execution. Transitions the agent from paused to active state. |
 | `fn_agent_create` | Create a new non-ephemeral agent. |

packages/cli/src/extension.ts

@@ -2610,7 +2610,7 @@ export default function kbExtension(pi: ExtensionAPI) {
     name: "fn_milestone_update",
     label: "fn: Update Milestone",
     description:
-      "Update an existing milestone's title, description, or acceptance criteria. " +
+      "Update an existing milestone's title, description, or acceptance criteria (the structured pass/fail bar, distinct from verification's free-form how-to-confirm notes). " +
       "Partial patches leave untouched fields intact.",
     promptSnippet: "Update an existing mission milestone",
     promptGuidelines: [