Fix all flake8 violations across the codebase

CODE STYLE FIXES:
- Fixed line length violations by breaking long lines appropriately
- Removed unused imports (os from installer.py and registry.py)
- Removed unused variables (package_name in _create_pyproject_toml)
- Fixed f-string usage (removed f-strings without placeholders)
- Fixed import organization and removed redundant imports
- Added missing newlines at end of files

SPECIFIC FIXES:
- cli.py: Split long option line, fixed f-string usage, added newline
- installer.py: Removed unused imports/variables, fixed line breaks, improved validation logic
- registry.py: Removed unused import/variable, broke long condition line
- test files: Removed unused imports, fixed long assertion lines, added newlines

All 24 tests still pass and flake8 now reports no violations.
Code is now compliant with PEP 8 style guidelines.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/kaizen_agentic/registry.py

@@ -1,6 +1,5 @@
 """Agent registry and management functionality."""
 
-import os
 import re
 import yaml
 from pathlib import Path
@@ -106,10 +105,10 @@ class AgentDefinition:
     def _determine_category(name: str, content: str) -> AgentCategory:
         """Determine agent category based on name and content."""
         name_lower = name.lower()
-        content_lower = content.lower()
 
         # Project management agents
-        if any(keyword in name_lower for keyword in ['todo', 'changelog', 'contributing', 'project']):
+        project_keywords = ['todo', 'changelog', 'contributing', 'project']
+        if any(keyword in name_lower for keyword in project_keywords):
             return AgentCategory.PROJECT_MANAGEMENT
 
         # Testing agents
@@ -288,4 +287,4 @@ class AgentRegistry:
             "comprehensive": [
                 agent.name for agent in self.list_agents()
             ]
-        }
+        }