Fix stale pending user-input after session restart

[claude-do]

Summary

This promotes the pending-question work we validated on dev.

• add a mobile-visible user-input debug panel for tracing pending question flow
• stop recovering provider sessions when responding to pending user input
• fail fast with a clear stale-question message after session restart
• clear stale pending question UI on the web so normal chat becomes usable again
• document the Bun PATH/Turbo typecheck gotcha in AGENTS.md

Root Cause

Pending user-input request IDs are session-local and live only in the original provider session's in-memory request map. After a service restart, the thread can still show the pending question in persisted activity state, but the original request ID is no longer answerable.

Before this change, respondToUserInput allowed provider-session recovery, which made the server attempt to answer a stale request on a different session. That produced Unknown pending user input request and left the thread stuck on the question UI.

Fix

• set allowRecovery: false for respondToUserInput
• return a clear stale-session validation error instead of attempting recovery
• map stale user-input failures to a user-facing recovery message in orchestration
• treat stale user-input failures as closed prompts in session derivation
• show a Question expired toast and restore access to normal chat

Validation

• bun fmt
• bun lint
• bun typecheck
• targeted Vitest coverage for provider service, provider command reactor, and web session logic

Related

• Robust fix for stale pending questions after session restart: detect mismatch and re-ask automatically #1
• Diagnosis: stale pending user-input requests break after provider session restart #2
• [BUG] Stale agent questions after app restart pingdotgg/t3code#469
• QnA prompts persist after app restart but can no longer be answered, session breaks pingdotgg/t3code#503
• Bug: pending plan questions become stuck after app restart pingdotgg/t3code#856
• Expire stale pending plan questions on cold app startup pingdotgg/t3code#862