D327 — The plan-implementer subagent is a thin per-client wrapper over the implement-issue skill

Decision. The repository ships one coding subagent, plan-implementer, in each client's own project format — .claude/agents/plan-implementer.md, .codex/agents/plan-implementer.toml, .kiro/agents/plan-implementer.json — with the same text. That text does not restate the procedure: it points at the "Canonical mandate" in .agents/skills/implement-issue/SKILL.md §2 and fixes only the defaults a coordinator would otherwise repeat in every mandate (stay in the worktree, never merge, stop on a stale plan, a ≤10-line report).

Why. Landing a batch of plans spawns one subagent per plan; with the mandate inline, every spawn carries the same long prompt and every report comes back in a different shape. A named agent shrinks the mandate to "issue #N, worktree X" and makes the report predictable. The cost is three files that must say the same thing, because no client reads another's agent format and a symlink cannot translate between Markdown, TOML and JSON.

Alternatives rejected. - Duplicating the mandate in each agent file: three copies of a procedure drift; the skill is already the single copy every client reads (D203). - A Cartographer-provisioned agent: provisioning serves KB artifacts to a user's machine, not a repository's own contributor tooling. - No agent, inline mandates only: works on every client, but costs the prompt on every spawn and leaves the report format to chance.

Model. Each definition pins a cheaper model than a coordinator usually runs on (Claude claude-sonnet-5-5, Kiro claude-sonnet-5.5, Codex terra-6): a plan issue already carries the design, the implementer executes it, and the coordinator's diff read is the quality gate. A model rename is a one-line change in each of the three files.

Consequences. A change to the mandate goes in the skill only. A change to the wrapper's defaults goes in all three files in the same commit. Antigravity has no documented project-local agent cell, so it keeps running the mandate inline.