feat(execute): one fresh helper per step, a controller on the diff, proof by command

/trekexecute no longer builds steps itself. Each plan step is built by one
fresh helper (new agent step-implementer: opus, effort high, no Agent tool)
and judged by one controller (a general helper on opus) that reads the
diff from a recorded start_sha and gives two verdicts, spec and quality.
Fix rounds go up, maximum 5: three fixes by a fresh helper, then back to
the plan, then to the operator. No cheaper-model ladder.

Proof is a command, never the model's word:
- lib/verification/red-proof.mjs: the step's test must fail at the start
  point and pass at the result (exit 0/1/2; 2 is never green). Reproduces
  the 02243c6 red-first measurement (6 of 55) through the shim fixture.
- lib/verification/test-integrity.mjs: the floor guard on the diff
  (removed asserts, skipped/deleted tests, muted checks, stubs, loosened
  thresholds). Over the last 150 commits: 149 clean, 1 real finding.
- lib/verification/done-line.mjs: DONE step N only when the test command
  exits 0.

Test first is required from plan_version 1.8. The test lock (K8) is used
through its CLI and reported as not-locked while it is missing. A run
stops only on a closed list of four classes; every other call goes into a
decision log with its cost if wrong. Without a passing run-the-thing
criterion the result is not verified end to end, never green.

Handover 5 gains optional per-step start_sha, report_path, red_proof,
test_lock, fix_rounds (0-5) and run-level decisions and end_to_end; old
progress files still validate and resume.

No isolation on the helper: measured, a worktree-isolated subagent
branches from the default branch (02f320f), not the executor's HEAD
(2c89a6b), so it would miss the earlier steps.

Borrowings credited in commands/trekexecute.md ## Credits (Superpowers,
ai-native-sdlc, Kiro, GitHub Spec Kit, agent-skills, Cherny).

Tests: 1411 -> 1457 (1455 pass, 0 fail, 2 skipped).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/profiles.md

@@ -73,10 +73,11 @@ native `effort:`.
 **Native `effort:` on agents.** Voyage sets the reasoning axis statically on
 selected agents, overriding the session level:
 
-- **Eight agents pin `high`:** the retrieval agents `task-finder`,
-  `git-historian`, `dependency-tracer`, `architecture-mapper`, and the
+- **Nine agents pin `high`:** the retrieval agents `task-finder`,
+  `git-historian`, `dependency-tracer`, `architecture-mapper`, the
   adversarial-reasoning agents `plan-critic`, `risk-assessor`,
-  `contrarian-researcher`, `review-coordinator`. No agent pins below `high`
+  `contrarian-researcher`, `review-coordinator`, and `/trekexecute`'s step
+  helper `step-implementer`. No agent pins below `high`
   (operator rule, 2026-10-05: standard is Opus 5.5 at effort High).
 - **All other agents:** unset → inherits the session's effort level (a session
   that nothing sets runs at its model's default — `medium` on Opus 5.5).