docs: Added documentation on the new architecture-reviewer agent

Agents.md

@@ -841,6 +841,11 @@ Coyote comes packaged with some useful built-in agents:
    `adversarial-review` skill). The post-implementation counterpart to `oracle`'s pre-implementation
    `plan-review`. Designed to be delegated to by `sisyphus` alongside `code-reviewer` (they cover different
    failure modes: quality vs. conformance).
+* `architecture-reviewer`: An on-demand architecture improvement scout (via the `codebase-design` skill). Scans a
+   codebase for deepening opportunities (e.g. shallow modules, leaked seams, missing locality, etc.), weighted by git-history
+   hot spots, presents candidates as a visual HTML (or markdown) report, then refines the chosen candidate into an
+   implementation-ready interface proposal via design-it-twice. Proposes only, never implements, and is NOT a
+   completion gate: invoke it when you want the codebase made deeper, more testable, and easier to navigate.
 * `architect`: A design-doc orchestrator for large-scale implementation jobs. Consumes a high-level design
    doc, decomposes it into a quality-gated plan (`gatekeeper` self-containedness gate + `oracle`
    plan-review) and ~1-engineer-day task files (via the `design-session` and `task-tracking` skills,