docs(standard): define clean portable contract v2

[ss-o]

Summary

• publish Zsh Plugin Standard 2 as one clean, portable contract for all plugin authors
• define consistent repository layout, namespace, configuration, public-surface, ownership, unload, and lifecycle-test requirements
• retire the shared Plugins registry and symbolic function-prefix schemes from current conformance
• extend the deterministic standard validator and regression tests for the versioned contract

Why

The existing page mixed portable requirements, historical z-shell conventions, and permissive alternatives. That made structurally different plugins appear equally conforming and left tools without one executable contract. Version 2 makes the portable standard itself the canonical architecture, without a z-shell-only compatibility layer.

Validation

• pnpm validate:plugin-standard
• pnpm validate:frontmatter
• pnpm validate:code-fences
• pnpm build:en
• trunk check community/00_contributing/03_zsh_plugin_standard.mdx scripts/validate-plugin-standard.mjs scripts/validate-plugin-standard.test.mjs (commit hook, passed)
• direct gitleaks scan, no leaks found
• git diff --check

Instruction impact

This changes the canonical public plugin-authoring contract. Follow-up pull requests will align organization instructions, patterns, templates, zsh-lint, ZUnit, and the zsh-fancy-completions pilot after this dependency is accepted.

Closes #848
Parent initiative: z-shell/.github#556

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