docs: complete codexskin authoring, import, and export workflow

[LouisDM]

What changed

• make direct .codexskin authoring the default Codex skin workflow
• add a first-screen prompt users can copy to request three visual directions without knowing templates, ports, or JSON
• document the complete skin.json + preview.png + assets + LICENSES source format and one-command packaging flow
• require preview.png to be a final 16:10 composite or real screenshot instead of a background-only image
• document every supported import entry point: Finder double-click, toolbar, ⌘O, drag-and-drop, and open
• document the real apply-state labels: “切换到此皮肤”“重新应用”和“重试切换”
• document permission-gated export through the toolbar or ⇧⌘E, the default filename, and private-skin lock behavior
• add SHA-256 plus export/re-import verification, including deterministic bytes, identical-package handling, same-version conflicts, and latest-version display behavior
• distinguish the manager runtime's verified 127.0.0.1:9340 connection from the declarative skin package, which never configures a port
• define a complete Git handoff: source files, licenses, wide/narrow screenshots, and distributable package/checksum when rights allow
• add the reusable $convert-to-codexskin Skill and read-only inspector for .command, CSS/image folders, ZIPs, legacy JSON, and copied runtimes
• reserve the lower-left Codex account row in the creation and conversion Skills
• require current templates to keep decorative title/theme/native sidebar content in Flex order -2 / -1 / 0
• require rendered geometry measurement with 0px theme/account overlap instead of screenshot-only approval
• add a guide contract test that parses the JSON example, verifies local links, and locks the UI/import/export/safe-area wording to the manager contract

Why

The old workflow made users understand implementation details before they could ask for a skin. It also produced a launcher, port, injector, CSS runtime, PID, and log lifecycle for every skin, then required another conversion step before the manager could import it.

The new flow starts with a useful product choice: three clear visual directions. After the user selects one, Codex creates the manager's restricted data-only format directly and keeps executable behavior in the manager.

Three concrete defects are prevented by this update:

1. A background-only manifest preview made 柳七月’s manager card differ from the applied skin. The guide now requires a final 16:10 composite preview.png.
2. Import was documented, but users could not discover how to export, why private skins were locked, or how to prove an exported package was portable. The guide now closes that loop with rights, checksum, and re-import checks.
3. Default-order sidebar pseudo-content occupied the same lower-left region as Codex’s native account control. Both Skills now treat the account row as a reserved interaction safe area and require measured 0px overlap.

The Git handoff rules also prevent incomplete cross-device work where only code or a guide reaches GitHub while the actual skin source, screenshots, package, or rights record remains on one Mac.

User impact

A new user can begin with:

查看 codex-cdp-skin-launcher.md，制作《沧元图》柳七月的 Codex 皮肤，先给我 3 个风格差异明显的方案选择。

A user with an existing non-.codexskin skin can install the repository Skill and say:

使用 $convert-to-codexskin，把“/我的皮肤路径”转换为 Codex Skin Manager 可导入的 .codexskin。先只读检查，不执行任何旧脚本；先告诉我可复用素材、需要转换的图片、模板映射、权利缺口和会被排除的文件，我确认后再打包和测试。

The guide now takes either path through authoring, packaging, import, application, export eligibility, checksum, and re-import. Public Git and Release uploads remain gated by redistributionAllowed and the recorded licenses.

Validation

• python3 -m unittest tests/test_codexskin_guide.py — 6 guide contract tests pass
• CodexSkinManager npm test — full suite passes
  • Swift: 72 tests, including deterministic export, private export refusal, round-trip import, version conflicts, and latest-version presentation
  • CDP engine: 6 tests
  • built-in skin packages, authoring packager, private package builder, and public Release assets all pass
• official Skill structure validation passes for convert-to-codexskin
• official Skill structure validation passes for redesign-codex-ui
• Python syntax compilation passes for the legacy-skin inspector
• staged diff passes git diff --check
• documented templates, tokens, asset slots, UI labels, shortcuts, package constraints, and commands were matched against CodexSkinManager 1.1.0 source

Rights and scope

This PR contains the reusable guide, Skills, tests, and safe tooling. Restricted 柳七月 source art, screenshots, and .codexskin are intentionally not added to this public repository. A skin may be publicly uploaded only when its recorded rights explicitly allow redistribution.