refactor: unify configuration governance for agent, core, and server

[Teingi]

This commit completes Phase 2 of the configuration governance refactoring
for the modules under responsibility (agent, core, intelligence, prompts).

Key changes:

• Remove direct os.getenv() calls from agent/agent.py, use AgentMemorySettings defaults
• Add timezone configuration support via unified config in core/memory.py
• Add programmatic timezone setting in utils/utils.py via set_timezone()
• Add to_config() methods and env_prefix to MemoryDecaySettings, PerformanceSettings, SecuritySettings
• Integrate performance, security, and memory_decay settings into PowermemSettings._COMPONENTS
• Unify API Server configuration to use settings_config() function
• Remove redundant arbitrary_types_allowed=True from ServerSettings

All configuration now follows the unified governance pattern:

• Component layers no longer directly read environment variables
• All settings use pydantic-settings BaseSettings with proper env_prefix
• Configuration is centralized through PowermemSettings.to_config()