Read the config file --config names

[cjbarth]

The bug

Program read the parsed option off the Commander command object:

getGrenConfig(props.cwd, program.config),

Commander has kept parsed options on opts() since v7, and this project is on v15. The
property is simply undefined, with no error and no warning, so --config and -c were
ignored and the run used whichever .grenrc* the directory happened to hold.

The failure is quiet in the worst way: gren prints the filename it loaded, so a run with
-c my-config.cjs reports Getting gren config from local file .../.grenrc.cjs and
carries on building a changelog from a configuration the user did not ask for.

The fix

Read the options from this.program.opts(), once, and use them for both the config
filename and the option merge.

A --config naming a file that is not there now stops the run. That has always been
getConfigFromFile's behaviour; nothing could reach it before.

Verification

Four tests, added first; three of them failed for the reported reason before the fix (the
fallback case passed, since falling back was all it ever did).

Run over this repository, generating to a scratch file with -c .grenrc-custom.cjs, where
the custom config sets a distinctive template.changelogTitle:

--- master: loaded .grenrc.cjs ---
# Changelog

## v5.1.1 (2026-09-16)

--- this branch: loaded .grenrc-custom.cjs ---
# Changelog

> Loaded from the config --config named.

npm test 135 passing; env -u GREN_GITHUB_TOKEN npx mocha 96 passing, 39 pending;
npm run lint clean apart from the pre-existing mocha/no-pending-tests warning in
test/Gren.spec.js. No change to lib/_options.js, so the README tables are unaffected.

🤖 Generated with Claude Code