feat(session-anchors): configurable budget scale + research-backed truncation fix

Remove 500-char assistant text truncation in anchor serializer — research
consensus (JetBrains NeurIPS 2025, SWE-agent, OpenDev ACC) is that agent
reasoning must never be truncated; only tool outputs get observation-masked.

Add AnchorBudgetScale type with 4 presets (Small=2K, Medium=6K, Large=12K,
Full=20K) and per-project range slider in SettingsTab. Remove Ollama-specific
warning toast — budget slider handles context limits generically.

v2/src/lib/components/Workspace/ProjectBox.svelte

@@ -157,7 +157,7 @@
       <ProjectFiles cwd={project.cwd} projectName={project.name} />
     </div>
     <div class="content-pane" style:display={activeTab === 'context' ? 'flex' : 'none'}>
-      <ContextTab sessionId={mainSessionId} projectId={project.id} />
+      <ContextTab sessionId={mainSessionId} projectId={project.id} anchorBudgetScale={project.anchorBudgetScale} />
     </div>
 
     <!-- PERSISTED-LAZY: mount on first activation, then toggle via display -->