fix: add missing vibe and agy agent configs to CommandRegistrar

Both agents are defined in AGENT_CONFIG in __init__.py but were absent
from CommandRegistrar.AGENT_CONFIGS in extensions.py, which meant the
extension system could never register commands for them.

- vibe (Mistral Vibe): .vibe/prompts, markdown format
- agy (Antigravity): .agent/commands, markdown format

Co-Authored-By: Claude (claude-opus-4-6) <noreply@anthropic.com>

Author: easonysliu

src/specify_cli/extensions.py

@@ -835,6 +835,18 @@ class CommandRegistrar:
             "format": "markdown",
             "args": "$ARGUMENTS",
             "extension": "/SKILL.md"
+        },
+        "vibe": {
+            "dir": ".vibe/prompts",
+            "format": "markdown",
+            "args": "$ARGUMENTS",
+            "extension": ".md"
+        },
+        "agy": {
+            "dir": ".agent/commands",
+            "format": "markdown",
+            "args": "$ARGUMENTS",
+            "extension": ".md"
         }
     }