Add long-running live-to-final replies RFC (#3464)

Merging the RFC as the agreed product contract for long-running-session assistant replies. Thank you @franksong2702! 🙏

It's docs-only (no code), well-structured, and gives the project a shared vocabulary for the follow-up implementation slices — in particular the honest terminal-state set (completed / cancelled / interrupted / compression-exhausted / tool-limit-reached / no-response / error, specific-wins-over-generic) and the live → settled → recovery/replay lifecycle. Nathan blessed merging it as the north-star contract.

docs/rfcs/README.md

@@ -46,6 +46,9 @@ First-time contributor RFCs should be discussed in an issue before opening a PR.
   — #2361 consistency rules for keeping transcript, model context, live streams,
   replay, compression, and session metadata coherent during active and recovered
   WebUI runs.
+- [`live-to-final-assistant-replies.md`](live-to-final-assistant-replies.md)
+  — #3400 product model for long-running assistant replies, live process prose,
+  tool activity, recovery, terminal outcomes, and the final-answer boundary.
 - [`canonical-session-resolution.md`](canonical-session-resolution.md) — #2361
   focused contract for resolving URL, query parameter, localStorage, sidebar,
   and compression-lineage session IDs to one canonical visible chat target.