fix(agents): remove unused is_dollar_skills_agent import

Leftover from replacing the inline ternary with get_invocation_prefix.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 65ef91d9-4c31-4f31-a009-ed2093fe7f28
Assisted-by: GitHub Copilot (model: claude-sonnet-4.6, autonomous)

Author: BenBtg

src/specify_cli/agents.py

@@ -15,7 +15,7 @@
 import yaml
 
 from ._init_options import is_ai_skills_enabled, load_init_options
-from ._invocation_style import get_invocation_prefix, is_dollar_skills_agent
+from ._invocation_style import get_invocation_prefix
 from ._toml_string import escape_toml_basic as _escape_toml_basic
 from ._toml_string import has_illegal_toml_control as _has_illegal_toml_control
 from ._utils import relative_extension_path_violation