feat: add configurable bash safe commands with layer support

[linw1995]

• Add BashConfig struct with safe_commands_path and safe_commands_mode (append/override)
• Implement BashConfigLayers to support global and workspace-specific safe command configurations
• Add configure_safe_commands() function to apply safe command layers in order (global → workspace)
• Update bash_executor.rs to use dynamic safe command rules with thread-safe OnceLock<RwLock>
• Support relative and absolute paths for safe command configuration files
• Add comprehensive tests for layer application order and mode combinations
• Integrate bash configuration loading into main config system with proper error handling

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.45963% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.22%. Comparing base (fad69e0) to head (25f19f0).
⚠️ Report is 5 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/agent/bash_executor.rs	79.48%	24 Missing ⚠️
src/config/mod.rs	38.88%	22 Missing ⚠️
src/agent.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #75      +/-   ##
==========================================
- Coverage   59.25%   59.22%   -0.04%     
==========================================
  Files          77       77              
  Lines       15488    15822     +334     
==========================================
+ Hits         9177     9370     +193     
- Misses       6311     6452     +141     

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