test: replace fixed time.Sleep with poll-until-ready in tmux tests

[tzone85]

Summary

`TestSendKeysRaw`, `TestSendKeysRaw_ThenEnter`, and `TestSendKeysAndCapture` captured the tmux pane after a fixed 500ms warmup + 300-500ms post-send sleep. Under CI load the `echo` hadn't completed before capture, so the assertion failed intermittently.

Fix

New helpers in `internal/tmux/send_test.go`:

• `waitForPaneReady` — polls until the pane shows any prompt-like text (session is up).
• `waitForPaneContains` — polls until the expected substring appears.

Both tick every 50ms against a 3-5s deadline. Fast laptops finish in well under the previous 500ms warmup; slow CI shells stretch transparently.

Test plan

• `go test ./internal/tmux/ -run "TestSendKeysRaw\$|TestSendKeysRaw_ThenEnter" -count=1` x10 — 10/10 pass
• `go test ./internal/tmux/ -count=1` x5 — 5/5 pass
• `go test ./... -count=1` — all 30 packages pass

Notes

• Local wall time: ~2.4s → ~1.6s on a fast machine because the poll finishes before the worst-case fixed sleep.
• The two `time.Sleep(300ms)` calls in `session_coverage_test.go` are intentionally left — those tests only assert `err == nil` and don't read specific content, so they're not a known flake source. Marginal optimisation, not a correctness change — out of scope.