feat: add Minimal Surface Alternatives design gate (v0.19.1)

[shinpr]

Summary

• Add a Gate-1 sub-process in technical-designer and technical-designer-frontend that requires each new field, state, mode, flag, or abstraction to be evaluated against subtractive alternatives via a 5-step procedure (Fix Requirements → Diverge → Compare → Converge → Record Rejected).
• Add a new principle in coding-principles, "Minimum Surface for Required Coverage", distinct from YAGNI: governs surface-area minimization at a fixed coverage point rather than time-axis judgment.
• Propagate the gate into document-reviewer (structured 4-sub-check) and code-reviewer (escalation criterion); update design-template with a per-element output section.
• Bump plugin version 0.19.0 → 0.19.1.

Changes

• skills/coding-principles/SKILL.md: add Principle 3 "Minimum Surface for Required Coverage", scoped to maintenance-surface-bearing elements (persistent state, public-contract or cross-boundary fields/props, behavioral modes/flags/variants, reusable abstractions, component splits).
• agents/technical-designer.md / agents/technical-designer-frontend.md: add Gate-1 sub-gate with in/out-of-scope criteria, precedence rule, resolution priority ordering, and self-contained 5-step procedure.
• skills/documentation-criteria/references/design-template.md: add Minimal Surface Alternatives section with per-element entry template; reverse "Known future requirements" to "Deferred possibilities" (positive form); scope "Extension points" to existing consumers.
• agents/document-reviewer.md: add Minimal Surface Alternatives check (scope trigger / section existence / per-element 4 sub-checks) and Gate-0 structural existence entry.
• agents/code-reviewer.md: add Escalation criterion for in-scope elements absent from Design Doc's Minimal Surface Alternatives section.
• Eligible requirements expanded from "user-visible requirements / ACs" to include "accepted technical constraints" (audit, data integrity, compatibility, security, performance, accessibility) — applied consistently across principle, designer, reviewer, and template.
• All cross-component references restricted to allowed dependency directions (recipe-skill → agent, recipe-skill → skill, agent → skill); reviewer agents are self-contained.

Test plan

• claude plugin validate passes on marketplace.json and all four plugin.json files
• pnpm sync:check confirms derived dev-workflows*/ directories are in sync
• pnpm check:skills-index passes
• pre-commit hook (sync-plugins / validate-plugins / check-skills-index) passes
• Manual: invoke technical-designer on a sample design and verify Gate-1 Minimal Surface Alternatives sub-gate fires on new field introduction
• Manual: invoke document-reviewer on a Design Doc missing the Minimal Surface Alternatives section and verify critical issue is raised

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