docs(02-01): complete link parser & idempotency plan

- Created LinkParser for unified URL parsing (B站/知乎)
- Created IdempotencyChecker for file existence checking
- All 23 new tests passing, 95% coverage
- Requirements PLAT-01, PLAT-02, PLAT-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: Phase 2 context gathered
-last_updated: "2026-03-13T09:43:10.102Z"
+stopped_at: Completed 02-01-PLAN.md
+last_updated: "2026-03-13T10:15:54.911Z"
 last_activity: 2026-03-13
 progress:
   total_phases: 4
   completed_phases: 1
-  total_plans: 3
-  completed_plans: 3
+  total_plans: 6
+  completed_plans: 4
   percent: 8
 ---
 
@@ -57,6 +57,7 @@ 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 |
+| Phase 02-platform-skills P01 | 5 | 3 tasks | 5 files |
 
 ## Accumulated Context
 
@@ -74,6 +75,8 @@ Recent decisions affecting current work:
 - [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
+- [Phase 02-platform-skills]: 使用 dataclass 定义 ParsedLink，短链解析使用 requests.head
+- [Phase 02-platform-skills]: 无法识别的 URL 返回 None 而不是抛异常，便于调用方处理
 
 ### Pending Todos
 
@@ -89,6 +92,6 @@ None yet.
 
 ## Session Continuity
 
-Last session: 2026-03-13T09:43:10.100Z
-Stopped at: Phase 2 context gathered
-Resume file: .planning/phases/02-platform-skills/02-CONTEXT.md
+Last session: 2026-03-13T10:15:54.910Z
+Stopped at: Completed 02-01-PLAN.md
+Resume file: None