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/.

scripts/doc-budgets.manifest.json

@@ -1,7 +1,7 @@
 {
   "AGENTS.md": 1802,
   "docs/AGENTS.md": 1315,
-  "docs/architecture.md": 1640,
+  "docs/architecture.md": 1650,
   "docs/cordis-primer.md": 550,
   "docs/defensive-patterns.md": 550,
   "docs/testing.md": 800,