feat: qctl implementer-backend seam (Part A, contract-first)

[rasatpetabit]

Summary

Wires a pluggable implementer backend into the masterplan plugin so execute
waves and github-coordination followers can eventually offload task
implementation to the petabit-sysadmin Qwen Work Fabric (pi/qctl worker
on the skynet1 GPU fleet) instead of always dispatching masterplan:mp-implementer.

This is Part A only — the masterplan-side seam — built contract-first.
The live binding is deferred until the fabric ships qctl (its task 11) and
gate.py (its task 12). Part B (the qctl results-contract) is doc-only and
captured in the design spec.

Hard invariant — held and verified: with the feature flag off (the default),
masterplan ships byte-for-byte identical to today. The flag is a strict
config.implementer.qctl.enabled === true; buildSeedState never emits an
implementer block, so existing bundles are unchanged and the absent block reads
as default-off.

What changed

• lib/routing.mjs — resolveImplementerBackend(task, config, env) + the
  kind-discriminated descriptor union ({kind:'agent'} default / {kind:'qctl', …}
  only when the flag is on). Sibling to routeTask (untouched).
• lib/wave.mjs — prepareWave/leanPayload stamp the per-task backend
  descriptor onto the wave payload.
• bin/masterplan.mjs — prepare-wave threads state.implementer ?? {} into
  the routing config.
• workflows/execute.workflow.js — implement(t) gains a single top-of-function
  guard: qctl → NotYetBound blocked digest (not a throw — a throw would null
  the pipeline item → silent vanish → re-dispatch loop); everything else → today's
  dispatch path, byte-identical.
• commands/masterplan.md §7 follow — resolves the lead-stamped descriptor
  instead of hard-coding mp-implementer; NotYetBound → mp:blocked (never re-adds
  mp:open, which would re-queue the unserviceable task into a claim/release loop).
• docs/config-schema.md — documents implementer.qctl.enabled (default false).
• docs/superpowers/{specs,plans}/ — the approved design + 7-task TDD plan.
• test/ — resolver coverage, byte-identical-default, flag-off-never-qctl, and
  NotYetBound-guard tests.

Also rides along: 04f8a54 (out-of-band Codex-review de-bloat — suppress the
gstack review skill dump + harden agents/mp-codex-reviewer.md).

Test Plan

• node --test test/ → 510 pass / 0 fail
• Codex defect pass — 2 P2s found, both fixed (6caaa16), 0 blocking
• Flag-off byte-identical invariant audited (4 code files additions-only;
  buildSeedState emits no implementer)
• Live binding (deferred): swap the NotYetBound guard + flip the flag once the
  Qwen Work Fabric ships qctl + gate.py

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