feat: generate template from .env file

[amahuli03]

Summary

Adds a dotenv template CLI command and generate_template() Python API to generate environment template files from existing .env files. Closes #618.

• Values are replaced with key names (e.g., SECRET_KEY=abc123 → SECRET_KEY=SECRET_KEY)
• Comments, blank lines, and export prefixes are preserved
• Supports inline directives:
  • # ::dotenv-template-preserve — keeps the line's value as-is (useful for non-sensitive defaults)
  • # ::dotenv-template-exclude — omits the line from the template
• Directives are stripped from output by default; --keep-directives flag retains them
• Output goes to stdout by default, or to a file via positional argument

Usage

Given a .env file:

# Print template to stdout
dotenv -f .env template

# Generate template file called `.env.template`, replacing and overwriting the file if it already exists
dotenv -f .env template .env.template

# Keep directive annotations in output
dotenv -f .env template --keep-directives

Python API

from dotenv import generate_template

template = generate_template(dotenv_path=".env")

Test plan

• Run python -m pytest tests/test_main.py tests/test_cli.py -k template -v — 14 tests covering value replacement, directives, keep_directives flag, file output, empty input, and error handling
• Manual test: create a .env with mixed directives and verify output matches expected template