[FEATURE] Shell integration for cx fix error capture

[mikejmorgan-ai]

Summary

The cx fix command expects errors to be in ~/.cx/last_error, but there is no shell integration to automatically capture command errors.

Current Behavior

$ some-command-that-fails
error: something went wrong

$ cx fix
# Error: No error captured. Run a command first or use: cx fix "error message"

Required Shell Integration

Bash (~/.cx/shell/bash_integration.sh)

• Capture stderr from last command
• Write to ~/.cx/last_error on non-zero exit
• Hook into PROMPT_COMMAND

Zsh (~/.cx/shell/zsh_integration.sh)

• Similar implementation using precmd hook
• Compatible with oh-my-zsh and powerlevel10k

Installation

cx setup shell  # Adds source line to ~/.bashrc or ~/.zshrc

Acceptance Criteria

• Bash integration captures stderr and exit code
• Zsh integration captures stderr and exit code
• cx setup shell installs integration
• cx fix reads captured error automatically
• Works with common error patterns (npm, cargo, python)
• Does not slow down shell noticeably
• BSL 1.1 header in shell scripts