Add plan mode and plan view with per-session agent context (#210)

* feat: add plan/build mode switching and Plan view tab

Add PlanView view and integrate plan tab into main layout
Introduce plan_enter and plan_exit tools with icons and status strings
Persist per-session agent selections in context to reduce UI flicker during mode switches

* feat: enchanced plan discovery checks in header and plan view

* fix(ui): align PlanView and Header with sessionDirectory logic

Remove dependency on context store in header and compute sessionDirectory from session or currentDirectory
Compute display and repo paths using sessionDirectory for PlanView and update path resolution
Poll plan content every 3 seconds to reflect changes without heavy retries

* feat(ui): improve header tab switching and add copy buttons for plan

Switch header tab navigation to central UI store and auto-switch when plan is unavailable
Show checkmark icon after copying file contents or path with auto-hide timeout
Extend plan view path resolution to prefer session directory when present

packages/ui/src/index.css

@@ -27,6 +27,12 @@ textarea[data-chat-input="true"]:hover {
   background: color-mix(in srgb, var(--accent) 70%, transparent);
 }
 
+.oc-plan-editor .cm-scroller,
+.oc-plan-editor .cm-content,
+.oc-plan-editor .cm-gutters {
+  font-family: "IBM Plex Sans", "SF Pro Text", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif !important;
+}
+
 :root.vscode-runtime textarea[data-chat-input="true"]::placeholder {
   color: color-mix(in srgb, var(--vscode-input-placeholderForeground, var(--muted-foreground)) 65%, transparent);
 }