Fold Agent Rules into the Tools forest

Antora component renamed from standalone agent-rules to tools module agent-rules (same pattern as Rules Manager, Switchyard, Ibex).

Docs URLs move from /agent-rules/ to /tools/agent-rules/. Hub playbooks drop agent-rules from the site-nav-tree forest allowlist; Tools remains the forest root.

Nav start label is Agent Rules (was Overview) so the Tools sidebar names the product.