Fix stale 'have agents' count in AGENTS.md (18 -> 17)

Addresses Copilot review on #362: agnix has 0 agents since v5.13.5, so
17 plugins have agents, matching the generated docs/reference/AGENTS.md
agents-counts. Add agnix to the skill/command-only list.

Author: avifenesh

AGENTS.md

@@ -178,7 +178,7 @@ agentsys                # Run installer
 <agents>
 ## Agents
 
-50 agents across 26 plugins (18 have agents; gate-and-ship is commands-only; axiom, banthis, skill-curator, and system-prompt-curator are skill/command-only; zig-lsp is config-only with no commands or agents; mojo and ada-spark are skill-only). Key agents by model:
+50 agents across 26 plugins (17 have agents; gate-and-ship is commands-only; axiom, banthis, skill-curator, system-prompt-curator, and agnix are skill/command-only; zig-lsp is config-only with no commands or agents; mojo and ada-spark are skill-only). Key agents by model:
 
 | Model | Agents | Use Case |
 |-------|--------|----------|