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

docs/conventions.md

@@ -104,6 +104,7 @@ The pipeline's nouns are fixed:
 | **cycle** | One round in the pipeline graph (`brainstorm → specify → planner → implement → audit → [fieldtest]`). NOT the top-level container. |
 | **iteration** | A sub-unit of a cycle. |
 | **milestone** | Tracker container spanning many cycles; closes only when complete AND functional (see `pipeline.md` § Milestone-close gate). |
+| **harvest sweep** | Batch shape draining accumulated settled single-issue backlog in one pass (see `pipeline.md` § Harvest sweep). |
 | **contract** | A single design-ledger entry. |
 
 When a project declares a glossary (in its CLAUDE.md project facts),