feat: add safe bash output redirection support

[linw1995]

Allow commands to redirect output to /dev/null and duplicate file descriptors (e.g., 2>&1) while blocking arbitrary file writes. Add validation and tests.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.24324% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.84%. Comparing base (e5977ab) to head (7c26afb).
⚠️ Report is 11 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/agent/bash_executor.rs	93.24%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #108      +/-   ##
==========================================
- Coverage   53.46%   51.84%   -1.63%     
==========================================
  Files          88       91       +3     
  Lines       21248    23448    +2200     
==========================================
+ Hits        11360    12156     +796     
- Misses       9888    11292    +1404     

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