feat: 添加架构约束和编码规范文档，增强项目文档的完整性

.ccw/specs/architecture-constraints.md

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+# Architecture Constraints
+
+## Schema Evolution
+
+- [compatibility] When enhancing existing schemas, use optional fields and additionalProperties rather than creating new schemas. Avoid breaking changes.
+- [portability] Use relative paths for cross-artifact navigation to ensure portability across different environments and installations.

.ccw/specs/coding-conventions.md

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+# Coding Conventions
+
+## Navigation & Path Handling
+
+- [navigation] When creating navigation links between artifacts, always compute relative paths from the artifact's actual location, not from an assumed location. Test path resolution before committing. (learned: 2026-03-07)
+- [schema] Always include schema_version field in index/registry files to enable safe evolution and migration detection. (learned: 2026-03-07)
+- [error-handling] When adding version checks or validation, always continue with degraded functionality rather than failing hard. Log warnings but don't block execution. (learned: 2026-03-07)
+
+## Document Generation
+
+- [architecture] For document generation systems, adopt Layer 3→2→1 pattern (components → features → indexes) for efficient incremental updates. (learned: 2026-03-07)
+
+## Implementation Quality
+
+- [validation] Path calculation errors are subtle and hard to spot in static review. Always verify path resolution from the actual file location, not from documentation. (learned: 2026-03-07)
+- [implementation] Declaring "add X to Y" in documentation is not enough - the actual logic must be implemented in the target files. (learned: 2026-03-07)
+- [clarity] Explicit instructions are better than implicit ones. Vague instructions like "Update _index.md files" should be made explicit (e.g., "Update sessions/_index.md"). (learned: 2026-03-07)