refactor(cli): drop dead fallback retry and followup-takeover paths

[rbalachandar]

Summary

Removes two CLI code paths whose last real consumer is gone now that the Runtime Host is the complete queue authority (per-entry queue ops, protocol epoch 40, #3544). Covers sections 1 and 2 of the audit; section 3 (runtime-kernel embedded queue API) is left for a separate PR pending the embedded-composition decision.

• Section 1 — fallback retry machinery (~650 lines): the only production driver (RuntimeHostMakaSessionDriverImpl) returns fallback solely when no sessionId exists, while every fallback producer (steer during a running turn, alt+enter queue) requires a live turn and therefore a session. Removes the retry timer loop, deferred-fallback state, turn-boundary flush, and the pending-bar merge, plus their tests.
• Section 2 — takePendingFollowup (~30 lines): the production stub is always null (the Host starts queued follow-ups atomically; returning text would double-submit), so the runner's re-queue/nextPrompt fold was unreachable outside test doubles. Removes the interface method, stub, consumer block, and doubles.

Kept as still needed (per the audit): trackEnqueue/settlePendingEnqueues (interrupt-path in-flight submit ordering), Host op queue.retract, sessions:steer/sessions:enqueue IPC, the session_busy fallback in sessions:send, and the QueueUpdateEvent steering/followup mirrors the pending bar renders.

Refs #3556

Verification

• npm run build — pass
• npm run lint (biome, 2652 files) — pass
• npm run format:check — pass
• npm run typecheck — pass
• packages/cli workspace suite: 412/412 pass
• Repo-wide grep: no remaining references to pendingFallback, takePendingFollowup, or the fallback retry helpers
• Not run: knip workspaces (apps/desktop, packages/ui — untouched by this PR), full multi-workspace npm test

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Claude Code — reviewed the removal against the issue inventory, verified kept/removed boundaries, ran verification; human contributor of record reviewed and owns the change.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[Astro-Han]

Verdict: NO-GO at exact head ad2d5a69cbd79e450688d7c40fa46b22da77c329. Simplify audit: removing the production always-null takePendingFollowup stub and its consumer is demand-free and sound, but the fallback producer is still reachable during first-session admission. A production-shaped delayed-prepare regression probe failed with actual prompts ['start'] versus expected ['start', 'must survive']; the current focused exact-head suites pass 249/249 because the fallback-specific coverage was deleted. Hosted exact-head test is green but does not exercise this window. One P1 inline; no other P0–P3 findings.

[Astro-Han]

[P1] Preserve first-session input until Session admission completes

runAgentTurn() sets turnRunning = true before preparePrompt() awaits #ensureSession(). On a new TUI Session, #createSession() has not yet assigned #sessionId, so Enter here (and Alt+Enter in the matching hunk below) calls the production driver's #enqueue(), gets { kind: 'fallback' }, and this callback now ignores that outcome after the editor was cleared. A production-shaped delayed-prepare probe observed this and retained only ['start'] instead of ['start', 'must survive']. Either expose queue availability only after Session identity exists or retain a durable fallback handoff, and keep a delayed session.create regression.