feat: hardcode wait time and default response as constants

Changes:
- Add DEFAULT_WAIT_SECONDS = 50 (replaces config.default_wait_seconds)
- Add DEFAULT_EMPTY_RESPONSE = 'call this tool... ' (replaces config.default_empty_response)
- Remove wait and empty response fields from WebUI Settings panel
  (only agent_stale_after_seconds remains configurable)
Rationale:
These values are fundamental to the 60s timeout mitigation strategy and
should not be user-configurable. The 50s wait with 5s keepalives is the
optimal balance for staying under the Copilot 60s wall-clock limit while
providing responsive progress feedback.

static/js/status.js

@@ -112,16 +112,12 @@ export function initStatus() {
 
 export function initConfig() {
   const form       = document.getElementById('config-form');
-  const waitInput  = document.getElementById('cfg-wait');
-  const emptyInput = document.getElementById('cfg-empty');
   const staleInput = document.getElementById('cfg-stale');
   const saveBtn    = document.getElementById('cfg-save');
 
   // Populate from state
   state.subscribe('config', (cfg) => {
     if (!cfg) return;
-    waitInput.value  = cfg.default_wait_seconds;
-    emptyInput.value = cfg.default_empty_response;
     staleInput.value = cfg.agent_stale_after_seconds;
   });
 
@@ -133,8 +129,6 @@ export function initConfig() {
 
     try {
       const cfg = await api.updateConfig({
-        default_wait_seconds: parseInt(waitInput.value, 10) || 10,
-        default_empty_response: emptyInput.value,
         agent_stale_after_seconds: parseInt(staleInput.value, 10) || 30,
       });
       state.set('config', cfg);