fix(tui): preserved b before /btw completion

Reserve the plain b shortcut only after /btw has a completed answer or a branch is already pending. Running, empty, aborted, and failed panels now leave the key for the composer, while completed-but-refused branches still consume it with an explanation.

Fixes #7474

packages/coding-agent/src/modes/controllers/btw-controller.ts

@@ -55,6 +55,19 @@ export class BtwController {
 		return this.#branchUnavailableReason() === undefined;
 	}
 
+	/** Whether plain `b` is currently reserved for a completed or pending branch action. */
+	handlesBranchKey(): boolean {
+		if (this.#branchInFlight) return true;
+		if (this.#activeRequest?.component.isBranchable() !== true) return false;
+		return (
+			this.#lastQuestion !== undefined &&
+			this.#lastReplyText !== undefined &&
+			this.#lastAssistantMessage !== undefined &&
+			this.#lastLeafId !== undefined &&
+			this.#lastSessionId !== undefined
+		);
+	}
+
 	#branchUnavailableReason(): string | undefined {
 		if (this.#branchInFlight) return "a branch is already in progress";
 		if (this.#activeRequest?.component.isBranchable() !== true) return "the answer is not ready";

packages/coding-agent/src/modes/controllers/input-controller.ts

@@ -250,7 +250,7 @@ export class InputController {
 			this.#btwBranchListenerInstalled = true;
 			this.ctx.ui.addInputListener(data => {
 				if (!matchesKey(data, "b")) return undefined;
-				if (!this.ctx.hasActiveBtw()) return undefined;
+				if (!this.ctx.handlesBtwBranchKey()) return undefined;
 				if (this.ctx.ui.getFocused() !== this.ctx.editor) return undefined;
 				if (this.ctx.editor.getText().trim()) return undefined;
 				void this.ctx.handleBtwBranchKey();

packages/coding-agent/src/modes/interactive-mode.ts

@@ -4844,6 +4844,11 @@ export class InteractiveMode implements InteractiveModeContext {
 		return this.#btwController.canBranch();
 	}
 
+	/** Reserves plain `b` only after /btw has a completed branch action to handle. */
+	handlesBtwBranchKey(): boolean {
+		return this.#btwController.handlesBranchKey();
+	}
+
 	handleBtwBranchKey(): Promise<boolean> {
 		return this.#btwController.handleBranch();
 	}

packages/coding-agent/src/modes/types.ts

@@ -418,6 +418,7 @@ export interface InteractiveModeContext {
 	handleBtwEscape(): boolean;
 	handleBtwBranchKey(): Promise<boolean>;
 	canBranchBtw(): boolean;
+	handlesBtwBranchKey(): boolean;
 	canCopyBtw(): boolean;
 	handleBtwCopyKey(): Promise<boolean>;
 	handleBtwBranch(