docs: add AI Agent Tooling manual section

Documents agents-init/agents-vault's concept, purpose, and complete
behavior as its own manual section (16), separate from the auto-generated
function reference: the AGENTS.md convention and CLAUDE.md retirement,
the AGENTS/ sub-repository (layout, versioning, hooks), per-directory
discovery and its four-state normalization, the two safety mechanisms
(discovery containment, deliberately-tracked-file protection), the
plans/specs/devlogs wiring, and the launch lifecycle.

Inserted before Attribution/License (now 17/18) rather than mid-document,
since that's the only placement that doesn't touch any of the manual's
prose cross-references to other section numbers. docs/fish-config.index
updated to match (new keywords, renumbered attribution/license entries;
agents-init and agy were already indexed to their own function-reference
entries and are left pointing there, not redirected to this new page).

docs/manual/18-license.md

@@ -0,0 +1,13 @@
+---
+title: License
+manTitle: 18. LICENSE
+sidebar:
+  order: 22
+helpKeywords:
+- license
+- licensing
+- agpl
+- copyright
+---
+
+<!-- README: License -->