docs(omp): add Oh My Pi platform to multi-platform and install docs

[VinciWu557]

配套 Trellis PR #307（feat(omp): add Oh My Pi platform support）的文档同步。

改动

新增 Oh My Pi（--omp）平台文档，en + zh 镜像同步：

• start/install-and-first-task.mdx：新增 --omp flag，平台数 16 → 17，Platform Configuration 表加 Oh My Pi 行
• advanced/multi-platform.mdx：平台数 16 → 17 + 平台列表加 Oh My Pi，Capability Matrix 加 Oh My Pi 列（extension-backed，与 Pi Agent 一致全 ✅），新增 "Oh My Pi" 小节

说明

Oh My Pi 与 Pi Agent 一样走 extension。与 Pi Agent 不同，Oh My Pi 没有 settings.json —— 原生 provider 会自动发现 .omp/ 下的所有子目录。

验证：4 个改动文件 prettier --check + markdownlint-cli2 均通过。

Summary by CodeRabbit

• Documentation
  • Updated quick start and multi-platform docs to include support for the new “Oh My Pi” platform.
  • Increased the documented supported platform count from 16 to 17.
  • Added platform configuration details, including the relevant directories and setup flow.
  • Expanded the platform compatibility matrix with “Oh My Pi” coverage.
  • Applied the same documentation updates to the Chinese translations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: a2fc88d9-1976-4482-8572-9a702931adda

📥 Commits

Reviewing files that changed from the base of the PR and between dcf2652 and b53d3ec.

📒 Files selected for processing (4)

• advanced/multi-platform.mdx
• start/install-and-first-task.mdx
• zh/advanced/multi-platform.mdx
• zh/start/install-and-first-task.mdx

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📝 Walkthrough

Walkthrough

The docs now add Oh My Pi as a supported platform in English and Chinese, update the platform count to 17, extend the capability matrix, add the --omp init flag, and document Oh My Pi’s configuration layout and runtime session path.

Changes

Oh My Pi platform documentation

Layer / File(s)	Summary
English docs update advanced/multi-platform.mdx, start/install-and-first-task.mdx	English docs add Oh My Pi to the platform list, update the count to 17, expand the capability matrix, add --omp, and document Oh My Pi setup and runtime paths.
Chinese docs update zh/advanced/multi-platform.mdx, zh/start/install-and-first-task.mdx	Chinese docs mirror the Oh My Pi platform list, count, capability matrix, init flag, and configuration details.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Poem

I hopped through the docs with a joyful twitch, 🐰
Oh My Pi joined the platform switch.
Seventeen paths now sparkle bright,
.omp/ glows in rabbit light.
Carrots for everyone—what a sight!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main doc update: adding Oh My Pi to the multi-platform and install guides.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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