feat: add tmux-based multi-terminal grid view

Launch Claude Code sessions inside embedded tmux panes instead of
separate Terminal.app windows. Sessions are tiled in a grid layout with
color-coded borders per project. Border flashing alerts when a session
goes idle and needs input. Ctrl+` switches between picker and grid view.

New modules:
- src/tmux/session-manager.ts — tmux session lifecycle
- src/tmux/ansi-parser.ts — ANSI escape code to cell grid parser
- src/tmux/capture.ts — polls tmux capture-pane for rendering
- src/tmux/input-bridge.ts — forwards keystrokes to tmux sessions
- src/components/terminal-view.ts — FrameBuffer renderable for panes
- src/components/session-grid.ts — tiled grid with flash effects

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/tmux/capture.ts

@@ -0,0 +1,53 @@
+export interface CaptureResult {
+  lines: string[]
+  cursorX: number
+  cursorY: number
+  width: number
+  height: number
+}
+
+export async function capturePane(sessionName: string): Promise<CaptureResult | null> {
+  try {
+    const [contentProc, infoProc] = [
+      Bun.spawn(["tmux", "capture-pane", "-t", sessionName, "-p", "-e"], { stdout: "pipe", stderr: "ignore" }),
+      Bun.spawn(["tmux", "display-message", "-t", sessionName, "-p", "#{cursor_x} #{cursor_y} #{pane_width} #{pane_height}"], { stdout: "pipe", stderr: "ignore" }),
+    ]
+
+    const [contentText, infoText] = await Promise.all([
+      new Response(contentProc.stdout).text(),
+      new Response(infoProc.stdout).text(),
+    ])
+
+    const [codeContent, codeInfo] = await Promise.all([contentProc.exited, infoProc.exited])
+    if (codeContent !== 0 || codeInfo !== 0) return null
+
+    const parts = infoText.trim().split(" ")
+    const cursorX = parseInt(parts[0]) || 0
+    const cursorY = parseInt(parts[1]) || 0
+    const width = parseInt(parts[2]) || 80
+    const height = parseInt(parts[3]) || 24
+
+    const lines = contentText.split("\n")
+    // Remove trailing empty line from split
+    if (lines.length > 0 && lines[lines.length - 1] === "") lines.pop()
+
+    return { lines, cursorX, cursorY, width, height }
+  } catch {
+    return null
+  }
+}
+
+// Hash for diffing - skip re-render if nothing changed
+let lastHash = ""
+
+export function hasChanged(lines: string[]): boolean {
+  // Simple fast hash: join first+last few lines + length
+  const h = lines.length + ":" + (lines[0] || "") + (lines[lines.length - 1] || "")
+  if (h === lastHash) return false
+  lastHash = h
+  return true
+}
+
+export function resetHash() {
+  lastHash = ""
+}