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.

HARNESS.md

@@ -119,7 +119,7 @@ is a codebase still navigable by `go doc` and `git log` alone, years from now, w
 **The style floor fails, it does not warn.** Forbidden package names, `init()`, importing `log`
 instead of `log/slog`, `panic()` outside `cmd/`, and `time.Now()` outside a `clock.go` are stated
 absolutely in `conventions.md`, so they exit non-zero. That last one exists because a Stage that reads
-the clock without declaring a cache validity window (ADR-0013) would serve staleness invisibly. A rule enforced as a suggestion teaches the agent to read every rule as one.
+the clock without an expiry would serve staleness invisibly. A rule enforced as a suggestion teaches the agent to read every rule as one.
 Softer signals — `fmt.Errorf` without `%w`, `interface{}`, nesting past 4, exported-and-referenced-
 once — stay advisory.
 
@@ -144,6 +144,8 @@ that describe it, in the same change.**
 - The gate is not optional. `scripts/hooks/pre-commit` runs it on every commit; enable once per clone
   with `git config core.hooksPath scripts/hooks`. `--no-verify` bypasses it, and the commit body should
   say why.
+- `docs/decisions.md` registers every ADR number ever used, entries and withdrawals alike, so a citation
+  can resolve to a decision or to a deferral but never to nothing.
 - `./scripts/verify.sh --list` names every gate that exists. A doc claiming enforcement is checkable
   against it in one command, and `/refresh-docs` checks it in both directions — a claimed gate that is
   missing, and a real gate nothing explains. Asserting a mechanism before it exists is the drift that