docs: add scope headers + Next: footers to all tour docs

Each of the eight tour docs (README, DESIGN, docs/CRYPTO,
docs/FORMATS, docs/SECURITY, crates/relicario-core/ARCHITECTURE,
crates/relicario-cli/ARCHITECTURE, extension/ARCHITECTURE) now
declares its scope in a blockquote under its H1 and ends with a
single-line "Next:" pointer to the next doc in the canonical
reading order: README → DESIGN → CRYPTO → FORMATS → SECURITY →
core → cli → extension.

Also trimmed README's mid-section "Architecture" stub to a one-
paragraph pointer at DESIGN.md (was duplicating cross-codebase
content and referencing a non-existent docs/architecture/ tree).

Renamed docs/CRYPTO.md's H1 from "Relicario — Architecture" to
"Relicario — Crypto Pipeline" to match the file's renamed scope.

Spec: docs/superpowers/specs/2026-05-30-doc-structure-redesign-design.md

crates/relicario-cli/ARCHITECTURE.md

@@ -1,5 +1,7 @@
 # Architecture: relicario-cli
 
+> **Audience:** contributors editing the CLI. This doc owns the CLI module map, the clap command surface, per-command key flows, session/unlock semantics, and helpers. **Does NOT own:** crypto, wire formats, or threat model (see [../../docs/CRYPTO.md](../../docs/CRYPTO.md), [../../docs/FORMATS.md](../../docs/FORMATS.md), [../../docs/SECURITY.md](../../docs/SECURITY.md)).
+
 ## What this crate is for
 
 The `relicario` binary is the platform layer for `relicario-core`: it adds
@@ -613,3 +615,7 @@ applies to `relicario-core` unit tests, not these CLI integration tests.
   is why every `cmd_*` that takes a `query: String` (get, edit,
   history, rm, restore, purge, attach, attachments, extract, detach)
   works the same way.
+
+---
+
+**Next:** [../../extension/ARCHITECTURE.md](../../extension/ARCHITECTURE.md) — the browser-side surface.