feat: add add-prompt-files input for appending file contents to prompts

[derekmisler]

Summary

Adds a new add-prompt-files input that allows users to include additional context files (like AGENTS.md or CLAUDE.md) as system messages in cagent prompts. The action passes these files to cagent using the --prompt-file flag, with cagent handling file resolution via directory hierarchy search.

Changes

• action.yml: Added add-prompt-files input parameter and shell logic to parse comma-separated file list and pass each as --prompt-file flag to cagent
• README.md: Documented the new input with usage examples, file resolution behavior, and integration with working-directory

Breaking Changes

This depends on docker/cagent#1618 which adds --prompt-file flag support. I'll want to bump the default version of cagent in this PR after that one is merged and released.

How to Test

• Add add-prompt-files: "AGENTS.md,CLAUDE.md" to a workflow using this action
• Verify the files are passed as --prompt-file arguments to cagent
• Confirm cagent includes the file contents as system messages in the prompt

[derekmisler]

/describe

[github-actions]

✅ PR description has been generated and updated!