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>

src/api/workflows.rs

@@ -10,19 +10,13 @@ use serde::Deserialize;
 use crate::AppState;
 use crate::agent::AgentEvent;
 use crate::db::{Workflow, PlanStep, Comment};
+use super::{ApiResult, db_err};
 
 #[derive(serde::Serialize)]
 struct ReportResponse {
     report: String,
 }
 
-type ApiResult<T> = Result<Json<T>, Response>;
-
-fn db_err(e: sqlx::Error) -> Response {
-    tracing::error!("Database error: {}", e);
-    (StatusCode::INTERNAL_SERVER_ERROR, e.to_string()).into_response()
-}
-
 #[derive(Deserialize)]
 pub struct CreateWorkflow {
     pub requirement: String,