fix(docs): finish the retirement sweep in docs/

`safety-gates` was red on #629 and it was right: `check-architecture-doc` found
eleven citations of make targets and scripts §6 deleted. My orphan sweep covered
make/, scripts/, .github/ and .githooks/ but never walked docs/, which is where
the architecture record lives.

docs/architecture/testing.md is REWRITTEN rather than patched. Every lane, script
and coverage floor it described is gone — patching the citations would have left
a document accurately describing a system that no longer exists. It now covers
component ownership across the Rust workspace and the TypeScript packages, the
public lanes, the wire-fixture parity proof and why Zig generates the fixtures,
and the 100% coverage bar. It also states plainly that the Zig tree is frozen and
unmeasured, and that redeploying it by hand is the rollback path.

AGENTS.md named `make test-integration`, `make memleak` and `make lint-governance`
as repository claims. Those are the operating instructions an agent reads before
it does anything, so a stale one there is worse than a stale architecture page.

Dimension 4.5 flips to DONE: #629 shows test-unit-rustd SUCCESS and
codecov/patch/rust-afd SUCCESS, neither in main's required-context set, which is
exactly what the Dimension asserts.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>