fieldtest: document the milestone fieldtest variant

Name the per-cycle vs milestone scopes; the milestone fieldtest feeds
the functional leg of the milestone-close gate (single-sourced in
pipeline.md).

fieldtest/SKILL.md

@@ -33,6 +33,33 @@ template, the source-isolation discipline (no reading under
 classification rules. This skill file only governs trigger,
 dispatch, and handoff.
 
+## Two scopes: per-cycle and milestone
+
+`fieldtest` runs at two scopes, both using the same fieldtester
+agent — the carrier picks the scope:
+
+- **Per-cycle fieldtest** (the default this file otherwise
+  describes): dispatched after a surface-touching cycle's audit,
+  carrier scoped to that cycle (`cycle_id` / `cycle_scope`).
+  Catches cycle-local friction early. Optional / orchestrator
+  judgement, as below.
+- **Milestone fieldtest**: the closing gate for a tracker
+  milestone. Carrier scoped to the milestone's promise
+  (`milestone_id` / `milestone_promise`); the fieldtester
+  derives curated end-to-end scenarios top-down from that
+  promise and proves the shipped implementation delivers it. Its
+  green status roll-up is required before a surface-touching
+  milestone may be closed.
+
+Both carriers are defined authoritatively under **Carrier
+contract** in `agents/fieldtester.md` (the milestone variant
+under its **Milestone-scope variant** sub-heading). The gate the
+milestone fieldtest feeds is defined once in
+`../docs/pipeline.md` § Milestone-close gate — not restated
+here. The milestone fieldtest is skippable on the same terms as
+the per-cycle one, lifted to milestone level: a milestone whose
+entire scope is internal (no user-visible surface) is exempt.
+
 ## When to Use / Skipping
 
 Triggers:
@@ -138,6 +165,9 @@ The orchestrator drives downstream:
   Common Rationalisations table, and the Red Flags list.
 - **Upstream gate:** `../audit/SKILL.md` runs first;
   fieldtest comes after a clean (or ratified) audit.
+- **Closing gate it feeds:** `../docs/pipeline.md`
+  § Milestone-close gate — the milestone fieldtest's green
+  roll-up is the functional leg of that gate.
 - **Cadence ordering:** fieldtest runs *before*
   `../docwriter/SKILL.md`; docwriter happens at a later,
   longer stability window.