refactor: migrate workflow system from 6-field nested to unified flat task schema

- Schema: add shared_context to plan-overview-base-schema, add pre_analysis/artifacts/inherited
  and polymorphic implementation (string|object with tdd_phase) to task-schema
- Producer: action-planning-agent outputs flat fields (description, depends_on, focus_paths,
  convergence.criteria, files, implementation, pre_analysis) + plan.json generation
- Orchestrator: plan.md/tdd-plan.md validate plan.json, task-generate-agent/tdd output dual-layer
- Consumer: code-developer/tdd-developer/test-fix-agent/universal-executor read flat fields
- Execute/review: read plan.json for execution strategy, use flat field paths
- Remove all migration notes referencing old field names

.ccw/workflows/cli-templates/schemas/plan-overview-base-schema.json

@@ -118,6 +118,28 @@
     },
 
 
+    "shared_context": {
+      "type": "object",
+      "description": "Project-level shared context (tech stack, conventions).",
+      "properties": {
+        "tech_stack": {
+          "type": "array",
+          "items": { "type": "string" },
+          "description": "技术栈列表"
+        },
+        "conventions": {
+          "type": "array",
+          "items": { "type": "string" },
+          "description": "编码约定"
+        },
+        "auth_strategy": {
+          "type": "string",
+          "description": "认证策略"
+        }
+      },
+      "additionalProperties": true
+    },
+
     "_metadata": {
       "type": "object",
       "required": ["timestamp", "source", "plan_type"],