feat(plan-task): add blast-radius non-negotiable

[bborbe]

Adds a sixth non-negotiable (fifth hard check) to plan-task § 5: when a task's subtasks push to a registry, deploy, mutate a cluster, or need a credential/secret, the task must name the external system AND the account written to.

Why: a credential requirement with no named target is a scope gap — the owner discovers what was automated at the secrets request, after the work has shipped.

Observed 2026-08-27 on the version-skew-guard task: a publish-on-tag CI was designed, PR'd, reviewed, merged and released with no objection. The owner questioned it ("Is the agent trying to push a docker image? I don't think that I want this") only when its Docker Hub secrets were requested — costing two reversal PRs for a net deletion. plan-task had passed the task: its existing checks cover SC/subtask/verify quality, never external blast radius.

Also updates the section preamble count (five → six checks, four → five hard).

[ben-s-pull-request-reviewer]

{
  "verdict": "approve",
  "summary": "PR adds a 6th hard non-negotiable 'Blast radius named' to plan-task, requiring tasks with registry pushes, deployments, cluster mutations, or credential needs to name the external system and account. The rule text is specific and well-grounded in an observed incident. CHANGELOG and command doc are consistent.",
  "comments": [],
  "concerns_addressed": [
    "correctness: preamble says sixth is soft but rule is hard — verified NOT an issue. The new 'Blast radius named' rule is correctly placed under **Hard:** (line 105). Counting: 1. Success Criteria defined, 2. Subtasks reach goal, 3. E2E verify present, 4. Subtask-goal alignment, 5. Blast radius named — all five hard. KISS ceiling is the sixth (soft). Preamble 'first five hard, sixth soft' is accurate.",
    "security: rule text specificity — confirmed adequate. Rule explicitly requires naming the external system AND the account written to, with a concrete example ('pushes docker.io/bborbe/<img> under the bborbe Docker Hub account'). The observed incident (publish-on-tag CI shipped Docker Hub push without owner awareness, costing two reversal PRs) is cited as motivation and correctly frames the failure mode."
  ]
}