harness: withdraw five pre-code ADRs to deferred ideas

Twenty-seven ADRs existed before a line of Go. Five specified the shape of
unbuilt mechanisms — cache validity, declared types, the settings cascade,
taxonomies, extras — which breaks the rule against abstraction before a
second concrete use, in prose where the counters cannot see it. They move to
ideas/deferred-decisions.md as recorded intent and return shaped by whatever
implements them.

Citations retargeted throughout; where one was decoration the rule now stands
on its own reasoning. Type declarations and the cascade drop to [spec] with
the MVP behaviour stated instead, so the first prompts have less to build.
conventions.md names http.ServeMux as the router, closing a hole that invited
hand-rolling a path splitter. The ADR gate now checks a number is registered
in the log rather than headed by an entry, so withdrawals resolve and invented
numbers still fail. Two architecture invariants corrected: identity no longer
implies a required language suffix, and the duplicated permalink clause is gone.

ideas/README.md

@@ -39,6 +39,7 @@ you, not the agent; `verify.sh` keeps it honest without reading it into context.
 
 ## Index
 
+- [deferred-decisions.md](deferred-decisions.md) — five pre-code ADRs demoted to intent; each returns when something implements it. **parked**
 - [specs-as-secondary-artifacts.md](specs-as-secondary-artifacts.md) — optional per-feature specs, derived by default, plus the named-test convention. **parked**
 - [engine-design-review.md](engine-design-review.md) — open design decisions for a multi-type site; items graduate to ADRs one at a time. **parked**
 - [token-conservation.md](token-conservation.md) — cut agent token cost without losing output quality. **parked**