docs: consolidate Codex onboarding

[chenncy]

Part of #1226

Summary

• Consolidate Codex onboarding documentation and remove duplicate setup guidance.
• Separate durable Memory from temporary Handoff workflows.
• Keep English and Chinese onboarding pages and navigation aligned.
• Keep the root README as an entry point and the plugin README focused on implementation details.

Validation

• uv run zensical build -s

AI usage

• Used documentation-writer and humanizer skills. Maintainer review is required.

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

This PR consolidates Codex onboarding documentation by removing duplicated setup steps from READMEs, adding dedicated pages that distinguish durable Memory from temporary Handoff workflows, and keeping the English/Chinese docs and navigation in sync (as part of #1226’s documentation restructure).

Changes:

• Update site navigation to include new “Memory and Handoff” explanation and “Hand off work in Codex” how-to pages (EN + ZH).
• Trim root and integration READMEs to point to the canonical docs instead of duplicating onboarding steps.
• Refresh Codex quickstart, docs overview, and install/config pages to reflect the Memory vs Handoff boundary and link to the new pages.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
zensical.toml	Adds EN/ZH nav entries for the new Memory/Handoff and handoff workflow pages.
README.md	Replaces duplicated Codex setup guidance with pointers to the canonical docs quickstart/overview.
integrations/codex/README.md	Keeps the plugin README implementation-focused and links to user docs for setup/troubleshooting.
docs/en/docs/tutorials/codex-quickstart.md	Reframes the tutorial around durable Memory across sessions and links to the new Handoff docs.
docs/en/docs/index.md	Updates the docs landing page “path” bullets, including Memory vs Handoff guidance and the new how-to.
docs/en/docs/how-to/install-and-run.md	Removes duplicated plugin command snippet and points to Configure Codex; clarifies readiness/doctor guidance.
docs/en/docs/how-to/configure-codex.md	Expands the conceptual section to cover automatic recall, Memory, and Handoff, with links to the new pages.
docs/en/docs/how-to/handoff-with-codex.md	New how-to describing the explicit Handoff workflow and when to keep it temporary vs durable.
docs/en/docs/explanation/memory-and-handoff.md	New explanation page defining Memory vs Handoff and when to use each.
docs/zh/docs/tutorials/codex-quickstart.md	Updates the quickstart narrative to focus on cross-session Memory maintenance and links to the new pages.
docs/zh/docs/index.md	Reworks the docs landing page “path” bullets, including Memory vs Handoff guidance and the new how-to.
docs/zh/docs/how-to/install-and-run.md	Removes duplicated plugin command snippet and points to Configure Codex; clarifies readiness/doctor guidance.
docs/zh/docs/how-to/configure-codex.md	Expands the conceptual section to cover automatic recall, Memory, and Handoff, with links to the new pages.
docs/zh/docs/how-to/handoff-with-codex.md	New how-to describing the explicit Handoff workflow and when to keep it temporary vs durable.
docs/zh/docs/explanation/memory-and-handoff.md	New explanation page defining Memory vs Handoff and when to use each.

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[chenncy]

Conflicts are resolved and uv run zensical build -s passes.

Could a maintainer approve the pending workflows and review the PR when available?
The CLA bot is also awaiting a status update.