feat: isolate Claude and Codex session sources

Persist platform_source across session creation, transcript ingestion, API query paths, and viewer state so Claude and Codex data can coexist without bleeding into each other.

- add platform-source normalization helpers and persist platform_source in sdk_sessions via migration 24 with backfill and indexing
- thread platformSource through CLI hooks, transcript processing, context generation, pagination, search routes, SSE payloads, and session management
- expose source-aware project catalogs, viewer tabs, context preview selectors, and source badges for observations, prompts, and summaries
- start the transcript watcher from the worker for transcript-based clients and preserve platform source during Codex ingestion
- auto-start the worker from the MCP server for MCP-only clients and tighten stdio-driven cleanup during shutdown
- keep createSDKSession backward compatible with existing custom-title callers while allowing explicit platform source forwarding

src/ui/viewer/components/ContextSettingsModal.tsx

@@ -136,7 +136,17 @@ export function ContextSettingsModal({
   }, [settings]);
 
   // Get context preview based on current form state
-  const { preview, isLoading, error, projects, selectedProject, setSelectedProject } = useContextPreview(formState);
+  const {
+    preview,
+    isLoading,
+    error,
+    projects,
+    sources,
+    selectedSource,
+    setSelectedSource,
+    selectedProject,
+    setSelectedProject
+  } = useContextPreview(formState);
 
   const updateSetting = useCallback((key: keyof Settings, value: string) => {
     const newState = { ...formState, [key]: value };
@@ -174,10 +184,23 @@ export function ContextSettingsModal({
           <h2>Settings</h2>
           <div className="header-controls">
             <label className="preview-selector">
-              Preview for:
+              Source:
+              <select
+                value={selectedSource || ''}
+                onChange={(e) => setSelectedSource(e.target.value)}
+                disabled={sources.length === 0}
+              >
+                {sources.map(source => (
+                  <option key={source} value={source}>{source}</option>
+                ))}
+              </select>
+            </label>
+            <label className="preview-selector">
+              Project:
               <select
                 value={selectedProject || ''}
                 onChange={(e) => setSelectedProject(e.target.value)}
+                disabled={projects.length === 0}
               >
                 {projects.map(project => (
                   <option key={project} value={project}>{project}</option>