Add --github to print GitHub Actions annotations

[Gulliby]

Before

envguard could print a human-readable report or JSON. In a GitHub Actions job the problems showed up only as log text, not as annotations on the run or the pull request.

After

envguard --github prints one workflow command per problem: missing and empty keys are ::error, extra keys are ::warning (errors with --strict). Nothing is printed when there is nothing to report. --json and --github cannot be combined (exit 2). Values are never printed, and file paths and messages are escaped as GitHub requires.

With A=,B=,C= in .env.example and A=1,C=,CI=true in .env:

::error file=.env,title=envguard::Missing key B (required by .env.example)
::error file=.env,title=envguard::Empty value for key C
::warning file=.env,title=envguard::Extra key CI (not in .env.example)

How

• src/format.ts: the annotation formatter and escaping.
• src/cli.ts: the --github option and its conflict check with --json.
• README.md: option row, a new GitHub Actions annotations section, exit-code note.
• tests/github.test.ts (new) and tests/cli.test.ts: 13 new tests; 147 pass in total.