feat: add --prompt-file CLI flag for including file contents as system context

[derekmisler]

Adds a new --prompt-file reusable flag that allows users to include additional files in the agent's system context, matching the behavior of add_prompt_files in agent YAML configs.

Usage

cagent exec agent.yaml "task" --prompt-file AGENTS.md --prompt-file CLAUDE.md
cagent run agent.yaml --prompt-file guidelines.md

Behavior

• Adds files as system messages (same as agent config add_prompt_files)
• Uses hierarchy search (finds files by traversing up directories)
• Checks home folder as fallback (~/)
• Merges with any add_prompt_files defined in agent config
• Deduplicates to avoid redundant context (saves tokens)

[github-actions]

Review Summary

✅ No issues found in the changed code!

The implementation correctly:

• Adds the --prompt-file CLI flag with proper flag registration
• Merges CLI prompt files with agent config files
• Deduplicates using the standard Go in-place filtering idiom
• Handles nil/empty slices safely
• Includes comprehensive test coverage

The code is well-structured and follows Go best practices.