fix(generate-docs): register zig-lsp in STATIC_PLUGIN_AGENT_COUNTS

The doc-freshness check (generate-docs.js --check) is the source of
truth for site/content.json's plugin stat. Without zig-lsp in the
static map, --check rewrote the stat back to 19, failing the freshness
test we run in CI.

Adds 'zig-lsp': 0 to the map (zero file-based agents, since zig-lsp
is config-only). STATIC_PLUGIN_COUNT now derives 20 from the keys.
STATIC_FILE_BASED_AGENT_COUNT unchanged because zig-lsp contributes 0.

Verified locally:
- node scripts/generate-docs.js --check exits 0
- __tests__/generate-docs.test.js: 64/64 pass
- __tests__/cli-subcommands.test.js: 13/13 pass

Author: avifenesh

scripts/generate-docs.js

@@ -418,7 +418,8 @@ const STATIC_PLUGIN_AGENT_COUNTS = {
   'web-ctl': 1,
   'skillers': 2,
   'onboard': 1,
-  'can-i-help': 1
+  'can-i-help': 1,
+  'zig-lsp': 0
 };
 const STATIC_PLUGIN_COUNT = Object.keys(STATIC_PLUGIN_AGENT_COUNTS).length;
 const STATIC_FILE_BASED_AGENT_COUNT = Object.values(STATIC_PLUGIN_AGENT_COUNTS).reduce((sum, count) => sum + count, 0);