feat: add fuzzy filter to TUI session picker

Users can now type to filter sessions in real-time:
- FilterableSelectList component wraps pi-tui's fuzzyFilter
- Matches against displayName, label, subject, sessionId
- j/k navigation, Enter selects, Escape clears filter then cancels
- Uses derivedTitle from previous commit for better display

Refs #1161

src/tui/theme/theme.ts

@@ -106,6 +106,12 @@ export const selectListTheme: SelectListTheme = {
   noMatch: (text) => fg(palette.dim)(text),
 };
 
+export const filterableSelectListTheme = {
+  ...selectListTheme,
+  filterLabel: (text: string) => fg(palette.dim)(text),
+  filterInput: (text: string) => fg(palette.text)(text),
+};
+
 export const settingsListTheme: SettingsListTheme = {
   label: (text, selected) =>
     selected ? chalk.bold(fg(palette.accent)(text)) : fg(palette.text)(text),