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/main.rs

@@ -1,6 +1,7 @@
 mod api;
 mod agent;
 mod db;
+mod kb;
 mod llm;
 mod exec;
 mod timer;
@@ -15,6 +16,7 @@ pub struct AppState {
     pub db: db::Database,
     pub config: Config,
     pub agent_mgr: Arc<agent::AgentManager>,
+    pub kb: Option<Arc<kb::KbManager>>,
 }
 
 #[derive(Debug, Clone, serde::Deserialize)]
@@ -56,9 +58,22 @@ async fn main() -> anyhow::Result<()> {
     let database = db::Database::new(&config.database.path).await?;
     database.migrate().await?;
 
+    // Initialize KB manager
+    let kb_arc = match kb::KbManager::new(database.pool.clone()) {
+        Ok(kb) => {
+            tracing::info!("KB manager initialized");
+            Some(Arc::new(kb))
+        }
+        Err(e) => {
+            tracing::warn!("KB manager init failed (will retry on use): {}", e);
+            None
+        }
+    };
+
     let agent_mgr = agent::AgentManager::new(
         database.pool.clone(),
         config.llm.clone(),
+        kb_arc.clone(),
     );
 
     timer::start_timer_runner(database.pool.clone(), agent_mgr.clone());
@@ -67,6 +82,7 @@ async fn main() -> anyhow::Result<()> {
         db: database,
         config: config.clone(),
         agent_mgr: agent_mgr.clone(),
+        kb: kb_arc,
     });
 
     let app = Router::new()