docs(01-03): complete error handler & config manager plan

- Structured error types with rich context (KBError, NetworkError, etc.)
- Configuration management via pydantic-settings from ~/.kb_env
- Automatic retry mechanism with tenacity (3 attempts, exponential backoff)
- All 100 tests passing
- Requirements REL-02, MNT-02, MNT-03 completed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.planning/STATE.md

@@ -4,14 +4,14 @@ milestone: v1.0
 milestone_name: milestone
 current_plan: 3
 status: executing
-stopped_at: Completed 01-01 WorkflowStateManager
-last_updated: "2026-03-13T09:19:17.560Z"
+stopped_at: Completed 01-03-PLAN.md (Error Handler & Config Manager)
+last_updated: "2026-03-13T09:22:34.916Z"
 last_activity: 2026-03-13
 progress:
   total_phases: 4
-  completed_phases: 0
+  completed_phases: 1
   total_plans: 3
-  completed_plans: 2
+  completed_plans: 3
   percent: 8
 ---
 
@@ -56,6 +56,7 @@ Progress: [█░░░░░░░░░] 8%
 
 *Updated after each plan completion*
 | Phase 01-foundation P01 | 5 | 3 tasks | 3 files |
+| Phase 01-foundation P03 | 8 | 5 tasks | 7 files |
 
 ## Accumulated Context
 
@@ -70,6 +71,9 @@ Recent decisions affecting current work:
 - [01-02]: 创建 pytest.ini 配置 pythonpath
 - [Phase 01-01]: Follow gsd-openclaw atomic write pattern for crash-safe state persistence
 - [Phase 01-01]: Return None instead of raising exception when no state file exists
+- [Phase 01-foundation]: 使用 pydantic-settings 从 ~/.kb_env 加载配置，env_prefix=KB_
+- [Phase 01-foundation]: 使用 tenacity 实现重试机制，3次重试+指数退避
+- [Phase 01-foundation]: 错误类包含 step_name, input_summary, suggestion 用于调试
 
 ### Pending Todos
 
@@ -85,6 +89,6 @@ None yet.
 
 ## Session Continuity
 
-Last session: 2026-03-13T09:19:17.559Z
-Stopped at: Completed 01-01 WorkflowStateManager
+Last session: 2026-03-13T09:22:34.914Z
+Stopped at: Completed 01-03-PLAN.md (Error Handler & Config Manager)
 Resume file: None