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/mod.rs

@@ -1,3 +1,4 @@
+mod kb;
 mod projects;
 mod timers;
 mod workflows;
@@ -9,16 +10,24 @@ use axum::{
     http::StatusCode,
     response::{IntoResponse, Response},
     routing::{get, any},
-    Router,
+    Json, Router,
 };
 
 use crate::AppState;
 
+pub(crate) type ApiResult<T> = Result<Json<T>, Response>;
+
+pub(crate) fn db_err(e: sqlx::Error) -> Response {
+    tracing::error!("Database error: {}", e);
+    (StatusCode::INTERNAL_SERVER_ERROR, e.to_string()).into_response()
+}
+
 pub fn router(state: Arc<AppState>) -> Router {
     Router::new()
         .merge(projects::router(state.clone()))
         .merge(workflows::router(state.clone()))
         .merge(timers::router(state.clone()))
+        .merge(kb::router(state.clone()))
         .route("/projects/{id}/files/{*path}", get(serve_project_file))
         .route("/projects/{id}/app/{*path}", any(proxy_to_service).with_state(state.clone()))
         .route("/projects/{id}/app/", any(proxy_to_service_root).with_state(state))