fix(schema): allow digits in compound trigger shortcuts

The COMPOUND_TRIGGER_PATTERN and description bracket regex only
accepted [A-Z]{1,3}, rejecting shortcuts like QD2 that include
digits. Update both regexes to accept [A-Z][A-Z0-9]{0,2} so
alphanumeric shortcuts work while still requiring a leading letter.

Add test fixture covering the alphanumeric shortcut case.

test/fixtures/agent-schema/valid/menu-triggers/compound-triggers.agent.yaml

@@ -29,3 +29,6 @@ agent:
     - trigger: H or fuzzy match on help
       description: "[H] Single-word compound trigger (1-letter shortcut)"
       action: help
+    - trigger: QD2 or fuzzy match on quick-dev2
+      description: "[QD2] Alphanumeric shortcut with trailing digit"
+      action: quick_dev2

tools/schema/agent.js

@@ -4,7 +4,7 @@ const { z } = require('zod');
 
 const COMMAND_TARGET_KEYS = ['workflow', 'validate-workflow', 'exec', 'action', 'tmpl', 'data'];
 const TRIGGER_PATTERN = /^[a-z0-9]+(?:-[a-z0-9]+)*$/;
-const COMPOUND_TRIGGER_PATTERN = /^([A-Z]{1,3}) or fuzzy match on ([a-z0-9]+(?:-[a-z0-9]+)*)$/;
+const COMPOUND_TRIGGER_PATTERN = /^([A-Z][A-Z0-9]{0,2}) or fuzzy match on ([a-z0-9]+(?:-[a-z0-9]+)*)$/;
 
 /**
  * Derive the expected shortcut from a kebab-case trigger.
@@ -100,7 +100,7 @@ function agentSchema(options = {}) {
               }
 
               // Validate that shortcut matches description brackets
-              const descriptionMatch = item.description?.match(/^\[([A-Z]{1,3})\]/);
+              const descriptionMatch = item.description?.match(/^\[([A-Z][A-Z0-9]{0,2})\]/);
               if (!descriptionMatch) {
                 ctx.addIssue({
                   code: 'custom',