Keep pending prompts safe across delayed starts and resets

[dnouri]

Summary

Keep locally owned prompts intact until Pi has authoritatively settled them.

• A delayed agent_start is no longer treated as an invisible no-turn command merely because 0.5 seconds elapsed. The fallback queries get_state; active turns remain pending, while an authoritative idle response releases the prompt.
• When an extension handles a prompt without starting an agent turn, the speculative local user turn is retracted and queued follow-ups can continue.
• Starting a new session now owns a guarded session transition immediately. It refuses unresolved prompt acceptance and queued follow-ups instead of silently discarding them.
• Deliberate reset while Pi is already streaming remains supported.
• Reset and state-refresh error paths release their transition guards reliably.

These are shared text-prompt lifecycle fixes extracted from the review of #271. That image-attachment PR is stacked on this one because it reuses the same ownership guarantees for image-bearing turns.

Verification

• make check — 1,662 tests, one compatibility skip, no unexpected results
• Full input, UI, and menu suites passed
• Focused no-turn, delayed-start, process-exit, and session-reset checks passed
• git diff --check passed