docs(rfc): plan mode to implemented — amendments folded on the way

The proposal survives contact with the code with three amendments, per
the RFCs-are-proposals rule. (1) A mode transition logs a
request/header-delta only when expressible: adding exit_plan_mode
resorts the canonical tool list, and a pure reordering has no delta
form, so entering plan mode logs the full fallback snapshot — the
attributability claim holds either way. (2) The proposed/ skeleton
converts to the implemented grammar: Proposal → Decision, the roadmap's
staging (now history) drops to the standing Deferred list, and
Acceptance criteria + Risks fold into Consequences (what holds, then the
accepted costs, including the ACP v2 mode-removal migration). (3) The
two recorded scenarios stay pending a with-key session, recorded in
Deferred.

The docs tail completes: the cookbook's plan-mode row upgrades from
sketch to the shipped package, architecture.md gains the ctx.modes
capability row (ceiling 1640 → 1650: a new capability service's table
row does not fit the old budget), and every reference repoints to
implemented/.

docs/architecture.md

@@ -31,6 +31,7 @@ The default distribution is a composition, not a hierarchy. `packages/core/` is
 | `ctx.web` | [`web/`](../packages/web/README.md) | search/fetch provider registries |
 | `ctx.compact` | [`compact/`](../packages/compact/README.md) | session-surface compaction |
 | `ctx.subagents` | [`subagent/`](../packages/subagent/README.md) | named delegation providers |
+| `ctx.modes` | [`mode/`](../packages/mode/README.md) | logged session modes (plan mode) |
 | `ctx.sessionPersistence` | [`session-persistence/`](../packages/session-persistence/README.md) | durable storage for session logs |
 
 ## Event Surface