SKILL.md Step 4: brief-composition rule covers marker SUPPRESSION but not DISPLACEMENT by a return-format contract

Goal

Extend the existing SKILL.md Step 4 brief-composition rule "A brief NEVER suppresses the implementation marker" (.claude/skills/issue/SKILL.md ~:2274) to cover the DISPLACEMENT variant, not only explicit suppression: an orchestrator brief that specifies an elaborate return contract ("Report back: the commit SHA, the diffstat, which tests you ran, …") without NAMING the epm:experiment-implementation marker duty crowds the marker out just as effectively as instructing the implementer to skip it, and manufactures the identical marker-shape code-review blocker plus an extra fix round.

Observed (task #1336, 2026-08-12, implementation round v20)

The orchestrator spawned experiment-implementer with a detailed brief that ended in an explicit "Report back: …" list enumerating the commit SHA, per-file diffstat, test results, smoke verdict, and unimplementable-spec disclosures. The brief never mentioned epm:experiment-implementation.

The implementer did excellent work — commit a0cda5cdb4a2a0350af75d85a0e59fd8b0873a88, +1056/−139, all three mandated mechanism changes, all 11 plan-named tests written and passing, smoke rc=0 — and returned all of the requested information as its final Agent text. It posted NO epm:experiment-implementation marker.

Consequence: code-reviewer returned FAIL with mechanical_contract_only: true and zero substantive blockers — both blockers were the absent round-v20 marker (mc1-impl-marker-absent-round-v20, mc2-smoke-run-evidence-not-in-marker; the reviewer probed the canonical channel 4× and checked the orphaned/deferred marker channels). The Step 5c-bis mechanical-contract strip could not be applied, because that strip is conditional on the implementer marker being present and conforming — and here the missing marker IS the blocker. One extra implementer round was needed purely to land the durable record of work already done.

Why the existing coverage does not close it

Three nearby passages each cover part of it:

• .claude/skills/issue/SKILL.md ~:2274-2279 — "A brief NEVER suppresses the implementation marker. Never instruct an implementer to 'post nothing' / skip its epm:experiment-implementation / epm:results marker — the code-review ensemble's mechanical contract KEYS on that four-section marker, so suppressing it manufactures a marker-shape blocker and an extra fix round (#1900)." This is scoped to an instruction to skip. A brief that is merely SILENT on the marker while loudly specifying a different return channel is not an instruction to skip, so the rule does not bite — yet the stated consequence ("manufactures a marker-shape blocker and an extra fix round") is reproduced exactly.
• .claude/skills/issue/SKILL.md ~:3581 — "spawn experiment-implementer with a brief naming the marker". This is the positive duty, but it is a clause inside a spawn-step sentence rather than a checkable brief-composition rule in the Step 4 bullet list where the suppression rule lives, and it carries no rationale a brief author would weigh against a competing return-format instruction.
• .claude/agents/experiment-implementer.md mandates the marker clearly and repeatedly (~:686 "Post the report as <!-- epm:experiment-implementation v<n> -->", ~:788, ~:799, the template at ~:811-897). The agent spec is not the gap. The gap is that a sufficiently specific orchestrator brief can functionally override a spec step without contradicting it — the brief is the proximate instruction the agent optimizes for.

Proposed fix (small, prose-only)

1. Extend the ~:2274 bullet with the displacement clause: a brief that specifies a return format NAMES the marker duty alongside it — e.g. "post your report as the epm:experiment-implementation marker at the next version (omit --version; the CLI derives max+1) AND return a short summary as your final text". State the mechanism in one clause: returned Agent text is NOT durable task state, so a return-only contract loses the record the code-review mechanical contract keys on.
2. Add the symmetric one-line reminder where the Step 4 brief is composed (the ~:3581 region), cross-referencing the extended bullet, so a brief author who reads only the spawn step still sees it.
3. No change to the strip logic, the marker schema, task.py, or any agent spec. The runtime behaviour is already correct; this is a brief-composition prevention fix.

Acceptance criteria

• The ~:2274 bullet covers BOTH shapes (explicit suppression AND silent displacement by a competing return contract) and states why returned text is not a substitute.
• The Step 4 spawn-step region cross-references it.
• uv run python scripts/workflow_lint.py (no flags) is no worse than its pre-change baseline (~15 pre-existing errors on main unrelated to this change — do not chase them; assert only that no new failure names an edited file).

Non-goals

• No change to the Step 5c-bis strip (its precondition — marker present and conforming — is correct as written).
• No new lint check. "Did the brief name the marker?" is not expressible from the surface files; the brief is composed at runtime.
• No change to experiment-implementer.md / implementer.md, which already mandate the marker.

Provenance

Surfaced by the /issue 1336 autonomous session at Step 5 of implementation round v20. Cost: one extra implementer round; no compute, no data loss. The orchestrator's own brief omission is the proximate cause — filed because the omission was invited by a rule that reads as fully covering the failure mode while covering only half of it.