feat(planning): design defaults name the principles; add lifecycle and reversibility #8334

Parent

Refs #8326 · Source: PLAN Phase 4, topic malleable-design-defaults

Depends on

• feat(planning): interview asks intent and quality requirements, reads consumer standards #8333

Outcome

/planning:design's "Design defaults" section names the principles and points at the planning plugin's design-principles reference. It reads consumer standards through the ladder. On configurability, it adds the per-tenant line, labeled vendor guidance: one mechanism, flags for temporary variation, entitlements per tier, and an owner and removal path for every option. NFRs already in the Brief count as settled. When the Brief has none, design reads the interview's NFR prompts file. Generalize-now-or-wait follows the interview's unattended rule.

A new "Lifecycle and reversibility" section, backed by an on-demand reversibility reference, adds:

• a one-way or two-way door call for each significant decision, with an ADR prompt for every one-way door;
• the consumer's existing style as the baseline, and a greenfield vertical slice as a default the user can override;
• design investment scaled by subdomain (core, supporting, generic; the three-way split is Vernon's and Khononov's, and Evans names core and generic);
• depth scaled by the Quality bar: line.

Key interfaces

From the plan's ## Design:

planning skills/design/reference/reversibility.md (new) | one-way/two-way doors, greenfield vertical-slice default, subdomain investment | proactive

planning skills/design | principle-named design defaults, lifecycle and reversibility section, per-tenant line | proactive

Intent: second variant = known | category | speculative; existing structure: <summary>; upstream: <summary or none>
Overrides bundled default <principle> per <standards file>

The skill text is pointer lines plus on-demand references (DA-C). A file from a sibling skill is cited in the ${CLAUDE_PLUGIN_ROOT}/... form, because that is the form the skill gate's Check 5 resolves for a sibling-skill path.

Done when

• grep -c "design-principles.md" on the design SKILL.md is ≥ 1, and the design skill's reversibility reference exists.
• Eval cases (case.yaml dirs under the planning plugin's evals/): an outcome case first (a task whose obvious design puts one fact in several files, graded on whether the design places it once); a one-way door gets an ADR prompt; a prototype skips reversibility and names it as deferred; an NFR from the Brief is not re-asked. Run the suite on main's plugin and on the branch's plugin; drop any line that does not improve on main.
• After the commit, bash scripts/check-changed-skills.sh origin/main exits 0.
• A changelog fragment for the planning plugin.

Out of scope

• A full multi-tenancy design section and a full strategic-DDD step.
• Handoff, plan and plan-reviewer gating (next item).