refactor(tmux-subagent): split manager and decision-engine into focused modules

Extract session lifecycle, polling, grid planning, and event handling:
- polling.ts: session polling controller with stability detection
- event-handlers.ts: session created/deleted handlers
- grid-planning.ts, spawn-action-decider.ts, spawn-target-finder.ts
- session-status-parser.ts, session-message-count.ts
- cleanup.ts, polling-constants.ts, tmux-grid-constants.ts

src/features/tmux-subagent/tmux-grid-constants.ts

@@ -0,0 +1,10 @@
+import { MIN_PANE_HEIGHT, MIN_PANE_WIDTH } from "./types"
+
+export const MAIN_PANE_RATIO = 0.5
+export const MAX_COLS = 2
+export const MAX_ROWS = 3
+export const MAX_GRID_SIZE = 4
+export const DIVIDER_SIZE = 1
+
+export const MIN_SPLIT_WIDTH = 2 * MIN_PANE_WIDTH + DIVIDER_SIZE
+export const MIN_SPLIT_HEIGHT = 2 * MIN_PANE_HEIGHT + DIVIDER_SIZE