Add the envguard-ci command

[Gulliby]

Requested by gulliby

Before. In a GitHub Actions step the usual call is envguard --github --strict, so that every problem, extra keys included, becomes an error annotation. There was no command that does this without flags.

After. The package has a second bin, envguard-ci, which behaves exactly like envguard --github --strict with the same options and exit codes (--json is rejected as with --github). bun run build now writes both dist/cli.js and dist/ci.js. With .env.example containing A=, B=, C= and .env containing A=1, C=, CI=true, node dist/ci.js prints and exits with 1:

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

How. New src/ci.ts inserts --github --strict into the arguments and loads ./cli. package.json gets the second bin entry and the build script bun build src/cli.ts src/ci.ts --outdir dist --target node. New tests/ci.test.ts has five tests; in tests/project.test.ts the assertion on the build script is updated and the new bin entry is checked. The README gets a "CI command" section. 170 tests pass; typecheck, lint and build are clean.