Remove queue code made dead by the Runtime Host per-entry queue operations #3556

Follow-up from #3544 (per-entry composer queue: queue.entry.retract / queue.entry.promote / queue.entries.reorder, protocol epoch 40). A simplification audit of that change surfaced three pre-existing code paths whose last real consumer is gone now that the Runtime Host is the complete queue authority. Each is a separate, independently revertable deletion; they should land as their own PR(s), not inside #3544.

1. CLI fallback retry machinery (~150 lines)

packages/cli/src/pi-tui-runner.ts: state.pendingFallback, deferFallback, scheduleFallbackRetry / retryPendingFallback / stopFallbackRetry, takePendingFallbackEntries(Settled), FALLBACK_RETRY_MS, the fallback branches in the submit path, the turn-boundary flush, and the pendingFallback merge in packages/cli/src/pi-transcript.ts (pending bar).

Evidence: the only production driver is RuntimeHostMakaSessionDriverImpl (sole factory createRuntimeHostMakaSessionDriver, sole entry runtime-host-tui-command.ts). Its #enqueue returns fallback only when there is no sessionId, while steerRunningTurn / handleAltEnter are only reachable with a turn running, i.e. a session must exist. Only test doubles ever return fallback. The MakaSessionDriver.steer/queueMessage contract permitting fallback is generality kept for a hypothetical embedded driver. Note: trackEnqueue / settlePendingEnqueues are NOT part of this — the interrupt path still needs their in-flight submit ordering.

2. CLI takePendingFollowup null stub and its consumer block (~30 lines)

packages/cli/src/session-driver.ts interface method, the always-null stub in runtime-host-session-driver.ts (its own comment says the Host starts queued follow-ups atomically, so returning text would make the TUI double-submit), and the consumption block in pi-tui-runner.ts that re-queues or folds the text into nextPrompt. Production always sees null; the only non-null implementations are test doubles, so tests exercise a path production can never reach.

3. runtime-kernel embedded message queue API (~60 lines plus tests) — needs one decision first

packages/runtime/src/runtime-kernel.ts: steer / queueMessage / drainFollowup / retractQueue (the coarse, \n\n-joining text queue), and the four pure pass-through methods in session-manager.ts. Production call sites: zero (desktop steers via Host turn.message.submit; the CLI drives the Host). The only consumers are session-manager.test.ts. assertEmbeddedMessageQueue already throws in hosted mode, and the single production composition always passes messageAuthority.

Decisive check before deleting: confirm that an embedded (Host-less) SessionManager composition is not a publicly documented/supported product shape (check ARCHITECTURE.md and docs/). If it is not, the block goes together with its tests. The SessionSteeringState lease/pull delivery used by hosted mode (bindMessage) is out of scope and stays.

Verified as still needed (do not remove)

• Host op queue.retract (retract-all): consumed by the CLI TUI interrupt and alt+up paths via driver.retractQueued(); the CLI has no per-entry UI.
• sessions:steer / sessions:enqueue desktop IPC: live consumers (workbar side-chat, renderer enqueue).
• The session_busy fallback in sessions:send: the race it covers still exists for skill/orchestration sends and side-chat.
• QueueUpdateEvent.steering / followup string arrays: the CLI pending bar reads them.