feat(agent): Add /tokens command for cumulative token usage tracking

Added /tokens slash command to TUI that displays session-wide token statistics.
Key changes:
- Fixed SessionManager to accumulate token usage instead of storing only last event
- Added cumulative token tracking to TUI renderer alongside per-request totals
- Implemented slash command infrastructure with /tokens autocomplete support
- Fixed file autocompletion that was missing from Tab key handling
- Clean minimal display format showing input/output/reasoning/cache/tool counts

The /tokens command shows:
Total usage
   input: 1,234
   output: 567
   reasoning: 89
   cache read: 100
   cache write: 50
   tool calls: 2

packages/tui/src/components/text-editor.ts

@@ -649,6 +649,8 @@ export class TextEditor implements Component {
 		// Check if we're in a slash command context
 		if (beforeCursor.trimStart().startsWith("/")) {
 			this.handleSlashCommandCompletion();
+		} else {
+			this.forceFileAutocomplete();
 		}
 	}
 
@@ -656,8 +658,34 @@ export class TextEditor implements Component {
 		// For now, fall back to regular autocomplete (slash commands)
 		// This can be extended later to handle command-specific argument completion
 		this.tryTriggerAutocomplete(true);
+}
+
+	private forceFileAutocomplete(): void {
+		if (!this.autocompleteProvider) return;
+
+		// Check if provider has the force method
+		const provider = this.autocompleteProvider as any;
+		if (!provider.getForceFileSuggestions) {
+			this.tryTriggerAutocomplete(true);
+			return;
+		}
+
+		const suggestions = provider.getForceFileSuggestions(
+			this.state.lines,
+			this.state.cursorLine,
+			this.state.cursorCol,
+		);
+
+		if (suggestions && suggestions.items.length > 0) {
+			this.autocompletePrefix = suggestions.prefix;
+			this.autocompleteList = new SelectList(suggestions.items, 5);
+			this.isAutocompleting = true;
+		} else {
+			this.cancelAutocomplete();
+		}
 	}
 
+
 	private cancelAutocomplete(): void {
 		this.isAutocompleting = false;
 		this.autocompleteList = undefined as any;