docs: add canonical session resolution contract

docs/rfcs/README.md

@@ -46,5 +46,8 @@ 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.
+- [`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.
 - [`turn-journal.md`](turn-journal.md) — Crash-safe WebUI turn journal for
   recovering interrupted chat submissions.