fix: size main pane using configured layout percentage

Main pane resize now uses main_pane_size instead of a hardcoded 50 percent fallback so post-split layout remains stable and predictable.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/tmux-subagent/action-executor.test.ts

@@ -86,6 +86,7 @@ describe("executeAction", () => {
 		expect(mockApplyLayout).toHaveBeenCalledTimes(1)
 		expect(mockApplyLayout).toHaveBeenCalledWith("main-horizontal", 55)
 		expect(mockEnforceMainPaneWidth).toHaveBeenCalledTimes(1)
+		expect(mockEnforceMainPaneWidth).toHaveBeenCalledWith("%0", 220, 55)
 	})
 
 	test("does not apply layout when spawn fails", async () => {