harness-runtime: replay history before opening the live attach

[nkurraDO]

What

attachToSession opened one connection whose catching_up (history) and live phases shared the wire — spliced apart server-side — with stdin read from the moment the streaming goroutine was launched, before any of that history had rendered.

Investigated how ui-aquarium's /events consumer does this (useSessionHistoryPager + useStreamSession in apps/ui-openharness): two sequential calls, not one — a finite replay_only history read first, then a live attach anchored strictly after that read's newest event (replayFrom: pager.newestEventId), with input held (ready = ... && !streamPending) until both settle. This PR gives doctl's interactive attach the same shape.

replayHistoryBeforeAttach reads history to completion synchronously, on the same goroutine that calls streamWithReconnect/runAttach right after:

• stdin is never read until history has fully rendered — this closes off MARSOHS-1026's user-visible symptom (a resent/duplicate turn rendering live, indistinguishable from new output, interleaved with whatever the user just typed) as a client-side defense, independent of OHP's own ingest-side dedupe fix.
• the live stream opens with ReplayFrom already seeded to the last history event's id — drainStream's existing cursor.set(ev.EventID) gives this for free — so there's no overlap window left for OHP to splice.

Best-effort on failure: a terminal error (auth, missing session) is reported once and returns, matching streamWithReconnect's own classifyStreamError handling — the live stream will report the same failure again on its own connect. A transient error falls through silently to the live stream's own catching_up window, which is the pre-existing behavior for that failure mode; this read isn't a new failure surface.

Doesn't touch agents logs / the codex proxy --replay path (#1903's backward paging) — different code path (one-shot batch read vs. this interactive attach seam).

Proof of testing

• Added unit tests

New: TestReplayHistoryBeforeAttach_RendersHistoryAndSeedsCursor, TestReplayHistoryBeforeAttach_TerminalErrorReportsAndReturns, TestReplayHistoryBeforeAttach_TransientErrorFallsThroughSilently, TestReplayHistoryBeforeAttach_CancelledContextIsSilent.

go test ./commands/... and go vet ./commands/... clean, including the full existing TestAttach*/TestStreamWithReconnect*/TestDrainStream* suite (unchanged).