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/fish-config.index

@@ -423,14 +423,21 @@ contributing=# 15. CONTRIBUTING
 contribute=# 15. CONTRIBUTING
 forge=# 15. CONTRIBUTING
 
-# ── Section 16: Attribution ───────────────────────────────────
-attribution=# 16. ATTRIBUTION
-credits=# 16. ATTRIBUTION
+# ── Section 16: AI Agent Tooling ──────────────────────────────
+agent=# 16. AI AGENT TOOLING
+agent-tooling=# 16. AI AGENT TOOLING
+agents.md=# 16. AI AGENT TOOLING
+claude-code=# 16. AI AGENT TOOLING
+antigravity=# 16. AI AGENT TOOLING
 
-# ── Section 17: License ───────────────────────────────────────
-license=# 17. LICENSE
-licensing=# 17. LICENSE
-agpl=# 17. LICENSE
-copyright=# 17. LICENSE
+# ── Section 17: Attribution ───────────────────────────────────
+attribution=# 17. ATTRIBUTION
+credits=# 17. ATTRIBUTION
+
+# ── Section 18: License ────────────────────────────────────────
+license=# 18. LICENSE
+licensing=# 18. LICENSE
+agpl=# 18. LICENSE
+copyright=# 18. LICENSE