add comment and code reorg

[wilsonwang371]

This pull request introduces several documentation improvements and updates to project rules, focusing on clearer guidelines for code and documentation maintenance, bilingual documentation practices, and technical doc updates for new and existing features. The changes help standardize documentation structure, improve discoverability, and clarify code-to-doc relationships across the repo.

Project rules and documentation standards:

• Added new project rules requiring that tests are not disabled for convenience and that code comments are updated in both Chinese and English when code is modified. (.trae/rules/project_rules.md)
• Introduced comprehensive documentation guidelines, covering naming conventions, structure, update triggers, and bilingual maintenance, with dedicated English and Chinese guideline docs. (docs/documentation-guidelines-en.md, docs/documentation-guidelines-zh.md) [1] [2]
• Added a new "Documentation Guidelines" section to the main documentation index for easier navigation. (docs/INDEX.md)

Technical documentation updates:

• Expanded the implementation docs section with a README in both English and Chinese, listing topics and enforcing naming conventions for bilingual docs. (docs/implementation/README-en.md, docs/implementation/README-zh.md) [1] [2]
• Added detailed design documentation for CChat default model selection, specifying configuration, routing, and error handling rules for model selection logic. (docs/implementation/cchat-default-model-selection-en.md)

API and code reference documentation:

• Clarified the separation of identity fields and metadata in ExecutionResponse, with explicit field documentation and code references in both languages. (docs/execution-response-fields-en.md, docs/execution-response-fields-zh.md) [1] [2]
• Updated architecture overview diagrams and code references to reflect the renaming of ExecutionRequest to ExecutionWorkItem in both English and Chinese docs. (docs/artifact-task-instance-architecture-overview-en.md, docs/artifact-task-instance-architecture-overview-zh.md) [1] [2]
• Updated function service integration docs to use InvokeRequest and submit_invocation instead of the previous request/manager pattern, in both English and Chinese. (docs/function-service-integration-en.md, docs/function-service-integration-zh.md) [1] [2]

Documentation index and discoverability:

• Added new entries to the documentation index for implementation notes, migration plans, and execution model refactoring, improving discoverability of technical docs. (docs/INDEX.md) [1] [2]

These changes collectively improve documentation quality, project maintainability, and developer onboarding.