feat: add Forgecode (forge) agent support

- Add 'forgecode' to AGENT_CONFIGS in agents.py with .forge/commands
  directory, markdown format, and {{parameters}} argument placeholder
- Add 'forgecode' to AGENT_CONFIG in __init__.py with .forge/ folder,
  install URL, and requires_cli=True
- Add forgecode binary check in check_tool() mapping agent key
  'forgecode' to the actual 'forge' CLI binary
- Add forgecode case to build_variant() in create-release-packages.sh
  generating commands into .forge/commands/ with {{parameters}}
- Add forgecode to ALL_AGENTS in create-release-packages.sh

src/specify_cli/agents.py

@@ -162,6 +162,12 @@ class CommandRegistrar:
             "format": "markdown",
             "args": "$ARGUMENTS",
             "extension": ".md"
+        },
+        "forgecode": {
+            "dir": ".forge/commands",
+            "format": "markdown",
+            "args": "{{parameters}}",
+            "extension": ".md"
         }
     }