Add global knowledge base with RAG search

- KB module: fastembed (AllMiniLML6V2) for CPU embedding, SQLite for
  vector storage with brute-force cosine similarity search
- Chunking by ## headings, embeddings stored as BLOB in kb_chunks table
- API: GET/PUT /api/kb for full-text read/write with auto re-indexing
- Agent tools: kb_search (top-5 semantic search) and kb_read (full text)
  available in both planning and execution phases
- Frontend: Settings menu in sidebar footer, KB editor as independent
  view with markdown textarea and save button
- Also: extract shared db_err/ApiResult to api/mod.rs, add context
  management design doc

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

doc/todo.md

@@ -9,10 +9,3 @@ template
 ---
 时间观察app
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-
-## 代码啰嗦/可精简
-
-- **agent.rs**：`NewRequirement` 与 `Comment` 分支里「设 final_status → 更新 DB status → broadcast WorkflowStatusUpdate → 查 all_steps → generate_report → 更新 report → broadcast ReportReady」几乎相同，可抽成共用函数（如 `finish_workflow_and_report`）；venv 创建/检查（create_dir_all + .venv 存在 + uv venv）两处重复，可抽成 helper。
-- **api/**：`projects.rs`、`workflows.rs`、`timers.rs` 里 `db_err` 与 `ApiResult<T>` 定义重复，可提到 `api/mod.rs` 或公共模块。
-- **WorkflowView.vue**：`handleWsMessage` 里多处 `workflow.value && msg.workflow_id === workflow.value.id`，可先取 `const wf = workflow.value` 并统一判断；`ReportReady` 分支里 `workflow.value = { ...workflow.value, status: workflow.value.status }` 无实际效果，可删或改成真正刷新。
-- **PlanSection.vue / ExecutionSection.vue**：都有 `expandedSteps`(Set)、`toggleStep`、以及 status→icon/label 的映射，可考虑抽成 composable 或共享 util 减少重复。