feat(fieldtest): bundle findings into collective issues; document the harvest sweep

Encode the tracker-growth practice ratified in the aura field run:

- fieldtest/SKILL.md — tracker filing defaults to bundling: one
  collective issue per fieldtest for related friction/spec_gap
  findings, consolidate-before-filing ahead of ANY new issue,
  per-issue filing only for real bugs (low-severity may wait in
  the collective issue). The fieldtester's per-finding report is
  unchanged; this is an orchestrator-side filing rule.
- docs/pipeline.md — new § Harvest sweep: the recognised batch
  shape for draining settled single-issue backlog (issue bodies
  as spec source, sequential implementer batches of 2-3, one
  non-removable independent whole-diff review, audit close,
  compact fieldtest under its normal skip rules). Registered as
  the specify/planner exception under § Skip rules.
- specify/SKILL.md, planner/SKILL.md — codify the sweep as a
  named alternative / skip case, per "skipping is codified per
  skill, not ad hoc".
- docs/conventions.md — register the noun in § Vocabulary.

closes #36

planner/SKILL.md

@@ -46,6 +46,11 @@ May be skipped when:
   design path — `brainstorm`, `specify`, and `planner`.
 - The work is a trivial mechanical edit (per the project's
   CLAUDE.md "trivial mechanical edits" carve-out).
+- The work is a harvest sweep — settled single-issue backlog
+  drained as a batch. The settled issue bodies stand in for the
+  plan, and the per-task review pair is replaced by one
+  whole-sweep diff review. See `../docs/pipeline.md`
+  § Harvest sweep.
 
 **Never skipped** for a standard iteration within an active
 cycle. "I know the cycle, plan from memory" is exactly the