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/index.html

@@ -158,16 +158,6 @@
       </div>
       <div class="panel-body">
         <form id="config-form">
-          <div class="config-field">
-            <label class="config-label" for="cfg-wait">Wait (sec)</label>
-            <input id="cfg-wait" class="input input--sm" type="number" min="0" max="300" value="10" />
-            <span class="config-hint">How long the tool waits for a new instruction before returning an empty response. Set exclusively here — agents cannot override this.</span>
-          </div>
-          <div class="config-field">
-            <label class="config-label" for="cfg-empty">Empty Response</label>
-            <input id="cfg-empty" class="input input--sm" type="text" placeholder="Leave blank for no response" />
-            <span class="config-hint">Default text returned when queue is empty</span>
-          </div>
           <div class="config-field">
             <label class="config-label" for="cfg-stale">Agent Stale After (sec)</label>
             <input id="cfg-stale" class="input input--sm" type="number" min="5" max="600" value="30" />