fieldtester: add milestone-scope carrier variant

Authoritative carrier contract for the milestone fieldtest: scoped to
milestone_promise, scenarios derived top-down rather than as a union of
per-cycle axes.

fieldtest/agents/fieldtester.md

@@ -72,6 +72,29 @@ If `axis_hints` is empty, infer from the cycle's spec under
 `paths.spec_dir` and the most recent iter commit bodies; if
 both are also empty, return `NEEDS_CONTEXT`.
 
+### Milestone-scope variant
+
+When dispatched as the **milestone fieldtest** — the closing
+gate for a tracker milestone (see `../../docs/pipeline.md`
+§ Milestone-close gate) — the carrier is scoped to the
+milestone's promise rather than one cycle's surface:
+
+| Field | Content |
+|-------|---------|
+| `milestone_id` | e.g. `milestone-7` |
+| `milestone_promise` | the milestone's stated goal / acceptance from the tracker (and its spec, if one exists) — what the work as a whole promises to deliver |
+| `commit_range` | `<milestone-start>..HEAD` |
+
+In this mode, Phase 1 derives 2-4 **curated end-to-end
+scenarios top-down from `milestone_promise`** — each scenario a
+realistic downstream task that, run end to end, demonstrates the
+milestone delivers what it promised. Do **not** assemble the
+scenarios as the union of per-cycle axes; the per-cycle
+fieldtests already covered axis-local surface. There is no
+`axis_hints` fallback in this mode: if `milestone_promise` is
+empty, infer it from the milestone's spec / tracker entry, and
+if that is also empty, return `NEEDS_CONTEXT`.
+
 ## The Iron Law
 
 ```