Make the REPL exit PTY test create its tmux session reliably #872

Story

The real-terminal exit regression starts its tmux PTY reliably, so a test failure describes REPL behavior rather than an allocator that never opened.

Current gap

On commit 7daceb5, the complete REPL gate intermittently failed before the product ran:

REPL exit: a real terminal, and nothing further typed
Error: tmux would not start a session for this row
at tmuxPty (packages/cli/tests/repl-exit-pty.test.ts:278:11)

The unchanged test passed immediately afterward in isolation. This makes the failure intermittent and leaves no product state to inspect.

Evidence

First run:

deno task test packages/cli/tests/repl-*.test.ts
FAILED | 109 passed (450 steps) | 1 failed (1 step)

Unchanged rerun:

deno task test packages/cli/tests/repl-exit-pty.test.ts
ok | 2 passed (4 steps) | 0 failed

The failure was observed locally while reviewing PR #871; there is no Actions run URL for the local process.

Acceptance

• The PTY fixture reports the underlying tmux startup output and condition that prevented session creation.
• Repeated or concurrent test execution does not intermittently fail before the REPL process starts.
• The regression still owns and joins the PTY child and leaves no tmux session or temporary directory behind.