docs(rfc): Transparent Stream activity display mode (#3820) (#3862)

* docs(rfc): add Transparent Stream activity display mode RFC (#3820)

Proposes Transparent Stream as an opt-in, chronological activity display
mode alongside the default Compact Worklog (#3400/#3401). Captures the
display-mode split agreed in #3820: each tool call as a first-class
chronological event, interleaved with reasoning/progress, with compact
previews, consistent across live, settled, and reload/replay paths.

Documents the asymmetry in the existing `simplified_tool_calling` toggle
(live-only, no settled/reload branch) and the three concrete integration
points so the follow-up can be sliced safely. Doc-only; no behavior change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs(rfc): refine Transparent Stream rollout scope

---------

Co-authored-by: Frank Song <franksong2702@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/rfcs/README.md

@@ -49,6 +49,10 @@ First-time contributor RFCs should be discussed in an issue before opening a PR.
 - [`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.
+- [`transparent-stream-activity-mode.md`](transparent-stream-activity-mode.md)
+  — #3820 opt-in display mode for power users who need a transparent,
+  chronological Thinking / progress / tool-call stream alongside the default
+  Compact Worklog model.
 - [`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.