feat: add -c/--config runtime arg for config file path

[TurtIeSocks]

Golbat has always read config.toml from the working directory. This adds a -c / --config flag so the file can live somewhere else, which matters for packaged installs (/etc/golbat/config.toml) and for running more than one instance from a single binary.

Both spellings work, since Go's flag package treats -x and --x the same:

golbat -c /etc/golbat/config.toml
golbat --config /etc/golbat/config.toml

Behavior

Without the flag nothing changes: config.toml in the working directory, and a missing file there is still fine, because the whole config can come from GOLBAT_* environment variables instead.

A path passed explicitly has to exist. If it doesn't, startup fails instead of quietly falling back to defaults. A typo'd --config that boots Golbat on stock settings looks alive but points at the wrong database, which is the worse of the two failures.

The old missing-file check

ReadConfig detected a missing file by comparing the error text against a hardcoded "open config.toml: no such file or directory". That stops matching as soon as the path becomes a variable, so every missing custom config would have printed a spurious warning. It is now errors.Is(err, fs.ErrNotExist).

Testing

config/reader_test.go covers the three cases: a missing default path is not an error, a missing explicit path is, and an explicit file actually loads.

• go build -tags go_json ./... clean
• go test -tags go_json ./config/ . both packages ok
• golangci-lint run ./config/... . 0 issues
• Built binary: -h lists both flags; -c /nope.toml and --config /nope.toml both fail with config file /nope.toml not found

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