Per-session workspace, tools only when workspace set

- Session has workspace field (absolute path, optional)
- ToolService stateless, receives workspace per call
- No workspace = no tools sent to API
- Removed hardcoded workspace from Program.cs
- Workspace field in session edit form

wwwroot/index.html

@@ -66,6 +66,8 @@
       <textarea id="sessionDescription" rows="3" placeholder="Beschreibung (optional)"></textarea>
       <label>Agent</label>
       <select id="agentSelect"></select>
+      <label>Workspace</label>
+      <input type="text" id="sessionWorkspace" placeholder="Absoluter Pfad zum Arbeitsverzeichnis (optional)">
       <div class="agent-form-actions">
         <button id="sessionSaveBtn">Speichern</button>
         <button id="sessionCancelBtn">Abbrechen</button>