feat: add support for template variables in prompt files through vars parameter

[FidelusAleksander]

This PR adds dynamic prompt templating support using Nunjucks templates and YAML variables, making the action much more flexible and reusable across different contexts.

✨ New Features

Template Variables Support

• New vars input parameter accepts YAML-formatted variables
• Templates are processed for all prompt inputs: prompt, prompt-file, system-prompt, system-prompt-file
• Full backward compatibility - existing workflows continue working without modification

Template Syntax

• Variable substitution: {{ variable }}
• Conditional logic: {% if condition %}...{% endif %}
• Loops: {% for item in items %}...{% endfor %}
• Filters: {{ text | upper }}, {{ items | join(', ') }}
• Nested objects: {{ user.name }}, {{ config.database.host }}

📋 Examples

Basic Variable Substitution

- uses: FidelusAleksander/prompt-action@v1
  with:
    prompt: "You are a {{ language }} expert translator. Translate {{ text }} to {{ target_language }}"
    vars: |
      language: Spanish
      target_language: English
      text: "Hola mundo"

GitHub Workflow Context Integration

- uses: FidelusAleksander/prompt-action@v1
  with:
    prompt: |
      Generate a PR description for {{ repo_name }}.
      Branch: {{ branch }}
      Author: {{ author }}
      Changes: {{ files_count }} files
    vars: |
      repo_name: ${{ github.repository }}
      branch: ${{ github.head_ref }}
      author: ${{ github.actor }}
      files_count: ${{ steps.count-files.outputs.count }}

Template Files with Loops and Conditionals

# .github/prompts/code-review.md
You are a {{ language }} expert specializing in {{ framework }}.
{% if include_tests %}
Please include testing recommendations.
{% endif %}

Review focus areas:
{% for focus in review_focus %}
- {{ focus }}
{% endfor %}

Code: {{ code }}

- uses: FidelusAleksander/prompt-action@v1
  with:
    prompt-file: .github/prompts/code-review.md
    vars: |
      language: TypeScript
      framework: React
      include_tests: true
      review_focus:
        - performance
        - security
        - best practices
      code: ${{ steps.get-code.outputs.content }}

Fixes #77.

[Copilot]

Pull Request Overview

Adds dynamic prompt templating support so users can supply a vars YAML block and have Nunjucks templates rendered for both prompt and system-prompt inputs while preserving backward compatibility.

• Introduce processTemplate() helper using js-yaml and Nunjucks
• Hook template processing into run() for prompt/prompt-file and system-prompt/system-prompt-file
• Update metadata, dependencies, tests, and docs to cover the new vars input

Reviewed Changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/template.ts	New processTemplate function parses YAML and renders via Nunjucks
src/main.ts	Calls processTemplate on both prompt and system-prompt inputs
package.json	Added js-yaml, nunjucks, and their type definitions
action.yml	Declared optional vars input
README.md	Documented templating feature with examples
tests/template.test.ts	Unit tests covering variable substitution, loops, conditionals, filters, and errors
tests/main.test.ts	Integration tests for templating in action run
tests/prompts/country-capital-prompt.md	Sample prompt template for country-capital test
tests/country-capital-response.json	JSON schema for the country-capital example
.github/workflows/test-action.yml	Updated test workflow to exercise templating and schema validation

Comments suppressed due to low confidence (2)

README.md:110

• The YAML snippet under "Basic Variable Substitution" is invalid: the prompt key should use a pipe (|) for a multi-line string rather than a quoted scalar on the next line. Update to:

prompt: |
  You are a {{ language }} expert translator. Translate the following text to {{ target_language }}: {{ text }}

    prompt:

tests/main.test.ts:642

• There is no test covering system-prompt-file templating with vars. Consider adding a test that mocks system-prompt-file and verifies it is rendered with the provided variables.

    it('should process template with prompt file', async () => {