docs(examples): add backend implementation guide

[vearne]

Document backend implementations in examples/backend and add links from examples indexes for easier discovery.

Summary by CodeRabbit

• Documentation
  • Added a new backend example guide for the examples catalog.
  • Expanded the examples list in both English and Chinese to include the new backend entry.
  • Documented backend usage, setup steps, supported backend options, agent tool integration, and error-handling behavior.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: e55077c7-b59f-4344-b0ef-a5a8e4a64435

📥 Commits

Reviewing files that changed from the base of the PR and between fd2501f and 36f4593.

📒 Files selected for processing (3)

• examples/README.md
• examples/README_zh.md
• examples/backend/README.md

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📝 Walkthrough

Walkthrough

Adds examples/backend/README.md documenting the pkg/backend virtual filesystem abstraction, its four implementations, and integration with Toolkit/agents. The two example index files (README.md and README_zh.md) each gain one new table row pointing to this example.

Changes

Backend Example Documentation

Layer / File(s)	Summary
New backend example README examples/backend/README.md	Introduces full documentation for the backend virtual filesystem abstraction: protocol design, four implementations (MemoryBackend, FilesystemBackend, LocalShellBackend, CompositeBackend), Toolkit/agent integration, in-band error conventions, quick-start, and related source pointers.
Index table entries examples/README.md, examples/README_zh.md	Adds a backend row to the Agent examples table in both the English and Chinese index files.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

A rabbit hops through virtual trees,
Memory, disk, and shell with ease,
Composite routes through forest paths,
New README carved in aftermath.
🐰 The filesystem docs now please!

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cursor/backend-docs

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