docs(memory): LRN-126 (split severs implicit data paths) + LRN-127 (SDD implementer git-ops discipline)

.claude/memory/learnings.md

@@ -1248,3 +1248,16 @@ rules:
 - **why**: a dual-use agent inherits ONE pinned model. If its two uses sit on different tiers (audit=big, execution=sonnet), the pin silently mis-tiers one of them. hotfixer dual-use is fine because BOTH its uses are execution (same tier); code-cleaner's would have straddled tiers.
 - **future application**: before making an agent dual-use, check both consumers are on the SAME tier. Audit/reflection consumer + execution consumer → split the routing (audit → big-model agent, execution → sonnet executor); never overload one pinned agent. Distinct from [[LRN-113]] (sweep ALL consumers on a pattern fix) — this is WHICH agent a consumer routes to, not whether you found them all.
 - **cousin**: [[BDR-066]] (model routing: reflection/audit big, execution sonnet), [[LRN-113]] (consumer-staleness sweep on a pattern fix).
+
+## LRN-126 — splitting a monolith agent severs every IMPLICIT data path; forward each consumed field through the handoff contract
+
+- **pattern**: wave-4 redaction-only split (client-handover-writer monolith → reflection-parent + sonnet doc-writer child) silently dropped 2 inputs the extracted STEPs consumed. `DEPLOY_HINTS` (detected in parent STEP 2, consumed by child STEP 14) + `--skip-seo` flag (parsed from `$ARGUMENTS`, gated child STEP 13) worked in the monolith by shared scope; after the split they were dead — never added to the PACKAGE. Child rendered a §8 without platform tailoring; `--skip-seo` became a silent no-op. Caught only by the opus whole-branch review, not the census.
+- **why**: in a monolith, `$ARGUMENTS`, detected vars, and STEP-N side-outputs are all in one scope — a later STEP reads them for free. The split turns that free read into a data path that MUST cross the parent→child contract explicitly. Every implicit read becomes a severed wire unless forwarded.
+- **future application**: when splitting an agent, enumerate EVERY field the child reads (grep child for its input vocabulary — `PACKAGE.`, bare var names, `$ARGUMENTS` flags) and diff against what the parent SETS before dispatch. Any child-consumed field the parent never populates = severed path = renders a hole or a silent no-op. A census that checks shape (model pin, gate-free) will NOT catch this — needs a data-flow read.
+- **cousin**: [[LRN-125]] (route consumer to right tier on a split), [[BDR-066]] (reflection/execution split), [[LRN-113]] (sweep ALL consumers). Distinct: 113/125 = WHICH agent/tier a consumer routes to; this = WHICH fields must cross the contract.
+
+## LRN-127 — SDD implementers must not run destructive git ops on files outside their task scope
+
+- **pattern**: a wave-4 fix-subagent ran `git checkout -- settings.json`, believing the model-value diff was a "test side-effect." It was the user's uncommitted `/model` → Opus switch ([[LRN-098]]), preserved all session. The checkout DISCARDED it — settings.json reverted to committed `claude-fable-5[1m]`. Implementer had no task-reason to touch settings.json; it acted on a file outside its diff.
+- **why**: a fresh implementer sees only its task + a dirty tree; it can't know which unrelated dirty files are intentional user state vs. cruft. Destructive git ops (`checkout --`, `reset --hard`, `clean -fdx`) on out-of-scope files are irreversible and erase context the implementer never had.
+- **future application**: dispatch briefs for SDD implementers / fix-subagents MUST bar destructive git ops outside the named task files. If the tree is dirty with unrelated changes, leave them — flag to controller, never revert. Controller owns cross-file git state; the executor touches only its own paths. Pairs with [[LRN-125]]/[[LRN-126]] as the "executor stays in its lane" family.