🏗️ refactor(kebab-mcp): fix ask tool production panic + mode default (fb-30)

Two issues from Task 7 review:

1. CRITICAL — call_tool "ask" arm called blocking ask handle from async
   context. OllamaLanguageModel::new builds reqwest::blocking::Client
   which creates+drops a tokio runtime → panic inside async. Fix:
   tokio::task::spawn_blocking wrap. Also applied preemptively to
   "search" arm (SqliteStore + Lance open are blocking IO too).

2. IMPORTANT — ask tool's retrieval mode hardcoded to Lexical (test
   workaround for provider="none"); CLI default is Hybrid. Fix: add
   `mode: Option<String>` field to AskInput, default Hybrid in handle,
   test passes mode=Some("lexical") explicitly to keep test functional
   on provider="none".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-mcp/tests/tools_call_ask.rs

@@ -52,6 +52,9 @@ async fn ask_tool_returns_answer_v1_with_refusal_on_empty_kb() {
             kebab_mcp::tools::ask::AskInput {
                 query: "what is the meaning of life".to_string(),
                 session_id: None,
+                // Test env uses provider="none" — Hybrid would hard-error on embedding.
+                // Pass Lexical explicitly so the test stays functional.
+                mode: Some("lexical".to_string()),
             },
         )
     })