[quality] deflake TestDeliverStartupKick_Delivered: gate readiness clock on actual pane render

[clubanderson]

Root cause

Same root cause as #4871, on the sibling test that PR didn't touch.

TestDeliverStartupKick_Delivered (src/pkg/agent/branches_coverage_test.go) calls paneInject(t, session, "goose is ready"), which types the marker into the tmux pane and sleeps a fixed 400ms, then immediately calls m.deliverStartupKick(...). Inside deliverStartupKick, readiness is polled via waitForCLIReadyForAgent, bounded by the TestMain-shrunk cliReadyTimeout. Under full-suite load (shared tmux server contention, ~282s for pkg/agent alone per the issue), tmux can take longer than 400ms to actually paint the marker, so the readiness clock starts before the pane has rendered it. waitForCLIReadyForAgent then times out, the kick is (correctly) dropped, and agent.LastKick stays nil — failing an assertion unrelated to the delivery logic under test.

Fix

Gate the readiness clock on the actual pane render instead of the fixed sleep, using the requirePaneShows helper already added in #4871 (tmux_coverage_test.go), which polls capture-pane until the marker is visible (15s test-only deadline). This mirrors exactly the pattern #4871 used to deflake the sibling test TestDeliverStartupKick_BobReceivesKick. No production timeouts or delivery-path assertions change — the test still asserts agent.LastKick is set on a matching launch generation.

Fixes #4865

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