docs: propose session modes — plan mode as logged per-agent policy state

A session mode is a named, logged, per-agent policy state: mode/set as a
log-only SessionEventMap member (todo/write shape), a foldMode over the log,
soft enforcement at system-prompt/assemble (filtered schemas + mode section,
auto-logged as request/header-delta), hard enforcement at tools/pre-execute
(deny-by-default against the mode allowlist), and a thin ctx.modes service
with turn-boundary pending-intent flush for user flips. One new product
package dsh-mode (packages/mode/mode, the approval-group shape); plan is the
only shipped definition.

Lands as ONE feature — a plan mode without a model-driven, approvable exit is
not a smaller version of the feature. The roadmap's two stages are build-and-
review order for a single stacked landing: the mode core, then exit_plan_mode
(ask-gated through the approval seam; the plan text rides the tool args as
the reviewable log artifact), the stdio readline answerer, and the ACP
session-mode surface (session/set_mode, current_mode_update, available
modes). Hard prerequisite: the approval seam lands first; the stack bases on
feat/sandbox-support meanwhile.

docs/rfc/INDEX.md

@@ -12,6 +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 |
 
 ### Simplification