docs: lead the RFC with plan mode; session mode is the mechanism

The deliverable is plan mode — retitle and reorder the proposal opener so
the feature leads and the logged session-mode state is its implementation
property. Fix the Codex citation in the boolean-planMode alternative: Codex
has plan mode (/plan); its ACP mode list carries its approval presets — two
different surfaces. Durable vocabulary (mode/set, ctx.modes, dsh-mode) stays
string-shaped so a second mode never renames logged event types.

docs/rfc/INDEX.md

@@ -12,7 +12,7 @@ Generated by `pnpm run gen-rfc-index` from the RFC tree — never edit by hand;
 | [Multiplex concurrent ACP sessions over one connection](proposed/feature/2026-06-14-acp-multi-session.md) | 2026-06-14 |
 | [Optional Code Mode — model writes TypeScript against an SDK of all tools](proposed/feature/2026-06-15-optional-code-mode.md) | 2026-06-15 |
 | [Pre-tool input rewrite — a consistent design](proposed/feature/2026-06-30-pre-tool-input-rewrite.md) | 2026-06-30 |
-| [Session modes — plan mode as logged per-agent policy state](proposed/feature/2026-07-07-session-modes-plan-mode.md) | 2026-07-07 |
+| [Plan mode — a logged per-agent session mode](proposed/feature/2026-07-07-session-modes-plan-mode.md) | 2026-07-07 |
 
 ### Simplification